THE  JAMES  K.   MOFFITT  FUND. 

LIBRARY  OF  THE  UNIVERSITY  OF  CALIFORNIA. 


GIFT  OF 

JAMES  KENNEDY  MOFFITT 

OF  THE  CLASS  OF  '86. 


Deceived  ,189     . 

Accession  No.      8  2  8 1 3     Class  No.  % 


THE 
EVOLUTION 

OF 

PLANTS 


CAMPBELL 


THEMAGM1L1AN 
COMPANY 


LECTURES 


ON    THE 


EVOLUTION   OF  PLANTS 


LECTURES 


ON   THE 


EVOLUTION   OF   PLANTS 


BY 


DOUGLAS   HOUGHTON   CAMPBELL,  PH.D. 

PROFESSOR   OF   BOTANY 
IN   THE   LELAND    STANFORD    JUNIOR    UNIVERSITY 


gorfe 
THE    MACMILLAN    COMPANY 

LONDON:  MACMILLAN  &  CO.,  LTD. 
1899 

All  rights  reserved 


COPYRIGHT,  1899, 
BY  THE  MACMILLAN  COMPANY. 


NorfoooU 
J.  S.  Cushing  &  Co.  —  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


PREFACE 

IN  the  present  volume  I  have  endeavored  to  present 
in  as  untechnical  a  manner  as  seemed  feasible  the 
more  striking  facts  bearing  upon  the  evolution  of  plant 
forms.  While  there  are  numerous  excellent  text-books 
in  which  most  of  the  statements  here  given  are  clearly 
presented,  there  is  none,  at  least  in  English,  so  far  as  I 
am  aware,  where  a  connected  account  of  the  develop- 
ment of  the  plant  kingdom  from  an  evolutionary  stand- 
point has  been  attempted.  Of  course  every  modern 
system  of  classification  is  based  upon  the  assumption  of 
a  genetic  connection  between  the  different  groups,  and 
must  take  into  account  the  origin  of  plant  forms ;  but 
these  phylogenetic  problems  are  necessarily  subordi- 
nated in  the  general  text-books.  Moreover,  these  text- 
books are,  for  the  most  part,  avowedly  prepared  for 
the  use  of  botanical  students  alone. 

It  seemed  to  the  writer  that  an  accurate,  if  some- 
what general,  and  not  strictly  technical,  statement  of 
our  present  knowledge  concerning  the  data  from  which 
the  genealogical  history  of  the  vegetable  kingdom  may 
be  traced,  might  be  of  interest  not  only  to  such  bota- 
nists as  have  not  concerned  themselves  specially  with 
this  phase  of  the  science,  but  also  to  zoologists,  and 
those  general  readers  who  are  interested  in  biological 
problems. 


82813 


vi  PREFACE 

The  substance  of  the  following  chapters  was  pre- 
sented in  the  form  of  a  course  of  lectures  at  Stanford 
University  during  the  past  year.  These  lectures  have 
been  carefully  revised,  and  a  number  of  drawings  pre- 
pared, which  it  is  hoped  will  be  helpful  in  elucidating 
the  text. 

Very  little  originality  can  be  claimed  for  the  matter 
presented,  beyond  its  arrangement.  The  writer  has 
availed  himself  freely  of  the  materials  accumulated 
through  the  labors  of  botanists  during  the  past  fifty 
years,  which  have  made  possible  such  a  general  presen- 
tation of  the  subject  as  has  been  here  attempted.  It  was 
intended,  at  first,  to  prepare  a  bibliography  of  the  more 
important  works  bearing  on  the  subject ;  but  it  was 
soon  evident  that  the  magnitude  of  a  bibliography, 
which  would  be  in  any  way  complete,  rendered  this 
impracticable. 

Most  of  the  drawings  have  been  made  by  the  author 
from  nature.  Where  these  have  been  borrowed,  dua 
acknowledgment  has  been  made. 

Special  thanks  are  due  my  colleague,  Dr.  G.  J. 
Peirce,  for  most  valuable  assistance  in  the  reading  of 
the  proofs. 

DOUGLAS  HOUGHTON   CAMPBELL. 

STANFORD  UNIVERSITY, 
November,  1898. 


CONTENTS 

CHAPTER  I 


INTRODUCTION 


PAGE 
1 


CHAPTER  II 
THE  CONDITIONS  OF  PLANT  LIFE 

CHAPTER  III 
THE  SIMPLEST  FORMS  OF  LIFE 31 

CHAPTER  IV 
ALG* 48 

CHAPTER  V 
FUNGI 80 

CHAPTER  VI 
MOSSES  AND  LIVERWORTS  (BRYOPHYTA)      .        .        .        .101 

CHAPTER  VII 
THE  FERNS  (PTERIDOPHYTA) 122 

CHAPTER  VIII 

PTERIDOPHYTA  (Concluded')     . 

vii 


yiii  CONTENTS 

CHAPTER   IX 

PAGK 

SEED  PLANTS  (SPERMATOPHYTA)  (GYMNOSPERM^E)     .        .     158 

CHAPTER  X 
ANGIOSPERIMLE  (MONOCOTYLEDONS) 177 

CHAPTER  XI 
DICOTYLEDONS 199 

CHAPTER  XII 

GEOLOGICAL  AND  GEOGRAPHICAL  DISTRIBUTION         .        .    220 

CHAPTER  XIII 
ANIMALS  AND  PLANTS 242 

CHAPTER  XIV 
INFLUENCE  OF  ENVIRONMENT 261 

CHAPTER  XV 

SUMMARY  AND  CONCLUSION    .  284 


EVOLUTION   OF   PLANTS 


CHAPTER  I 

INTRODUCTION 

WITH  the  acceptance  of  the  theory  of  evolution,  the 
question  of  the  origin  and  affinities  of  the  manifold 
forms  of  life  making  up  the  organic  world  becomes  of 
absorbing  interest  to  the  biologist,  and  the  hope  of 
solving  some  of  these  problems  has  been  the  great  in- 
centive to  much  of  the  most  brilliant  work,  both  of 
zoologists  and  botanists,  during  the  latter  half  of  the 
nineteenth  century. 

When  we  survey  the  vast  assemblage  of  living  organ- 
isms, the  thought  of  arranging  these  in  orderly  sequence 
seems  hopeless  ;  and  indeed  when  we  take  into  account 
how  many  forms  must  have  disappeared  and  left  no 
trace  behind,  it  must  be  admitted  that  the  task  is  one 
whose  completion,  if  ever  reached,  must  lie  in  the  dis- 
tant future.  Nevertheless  the  data  are  slowly  but 
surely  accumulating  through  the  efforts  of  biologists 
whose  patient  researches  are  constantly  adding  to  our 
knowledge,  both  by  the  discovery  of  new  forms  and 
by  a  more  thorough  examination  of  those  already 
known.  The  constant  improvement  in  the  technical 
appliances  for  research,  such  as  the  microscope  and 

B  1 


2  EVOLUTION    OF   PLANTS 

microtome,  improved  methods  of  staining,  etc.,  as  well 
as  the  extension  of  explorations  to  the  remoter  parts  of 
the  world,  have  all  contributed  to  these  advances  in 
knowledge,  and  have  been  fruitful  sources  for  new 
materials.  The  brilliant  results  already  attained  make 
it  reasonable  to  hope  that  others  of  equal  importance 
are  yet  to  follow. 

Very  much  remains  to  be  done,  and  any  conclusions 
based  upon  the  data  now  at  hand  must  be  subject  to 
change  as  new  facts  are  brought  forward  ;  nevertheless 
enough  is  at  present  known  to  warrant  an  attempt,  at 
least,  at  an  arrangement  of  the  larger  groups  of  plants, 
showing  their  mutual  affinities.  Some  of  these  groups, 
however,  like  the  Diatoms,  stand  very  much  by  them- 
selves, and  it  must  be  admitted  that  their  relationships 
with  other  plants  are  extremely  problematical. 

In  spite  of  the  almost  infinite  diversity  of  structure 
shown  by  plants  and  animals,  it  is  found  that,  only  a 
very  small  number  of  the  seventy  or  more  chemical 
elements  known  at  present,  enter  into  their  composition. 
Carbon,  oxygen,  hydrogen,  nitrogen,  and  probably  sul- 
phur are  always  present,  and  usually  at  least,  potassium, 
phosphorus,  calcium,  magnesium,  and  iron.  In  addition 
to  these  are  a  number  of  elements  usually  present,  but 
apparently  not  essential  for  the  manifestation  of  life. 
These  elements  are  combined  into  extraordinarily  com- 
plex substances  whose  exact  analysis  often  baffles  the 
chemist,  owing  to  their  excessive  instability.  Inasmuch 
as  none  of  the  elements  found  in  living  matter  are 
peculiar  to  it,  but  are  also  found  in  the  so-called  "  in- 
organic "  substances,  and  since  all  living  bodies  are 
directly  or  indirectly  dependent  upon  the  latter  for 


INTRODUCTION  3 

their  growth,  it  is  at  once  evident  that  any  attempt  to 
make  a  hard  and  fast  line  separating  organic  and  inor- 
ganic substances  must  necessarily  prove  futile. 

There  are,  however,  certain  properties  peculiar  to 
living  organisms  which  may  be  said  definitely  to  char- 
acterize them,  i.e.- the  power  of  spontaneous  movement, 
nutrition,  and  reproduction.  All  of  these  functions 
are  associated  directly  with  that  remarkable  substance 
protoplasm,  most  happily  designated  by  Huxley  the 
physical  basis  of  life.  So  far  as  ordinary  chemical  and 
physical  tests  go,  the  protoplasm  of  all  living  organisms 
is  much  alike  ;  of  course  this  does  not  imply  that  pro- 
toplasm is  a  definite  chemical  compound  such  as  starch 
or  sugar ;  it  is  rather  to  be  considered  as  a  mixture  of 
excessively  complex  and  unstable  substances,  more  or 
less  similar  in  the  elements  of  which  they  are  made  up. 

The  constituents  of  this  protoplasm  are  evidently 
very  unstable,  as  every  manifestation  of  life  in  the  liv- 
ing protoplasm  is  necessarily  bound  up  with  chemical 
changes  in  its  substance.  Where  the  protoplasm  is 
present  in  sufficient  quantity  to  be  handled  in  mass,  as 
in  those  curious  organisms,  the  Slime-moulds,  by-pro- 
ducts are  usually  present  which  interfere  with  an  accu- 
rate analysis. 

The  simplest  forms  of  life,  like  the  Bacteria,  often 
show  little  structure  beyond  a  mass  of  apparently 
homogeneous  protoplasm  surrounded  by  a  delicate 
membrane,  but  it  is  exceedingly  doubtful  whether  this 
extreme  simplicity  is  more  than  apparent,  owing  to 
the  excessively  minute  size  of  these  organisms.  The 
presence  of  a  nucleus,  or  at  any  rate  nuclear  substance 
in  bacteria,  is  by  no  means  improbable.  Among  ani- 


EVOLUTION   OF   PLANTS 


mals  the  so-called  "  Monera,"  which  were  formerly 
supposed  to  be  composed  of  structureless  protoplasm, 
are  now  known  to  possess  a  nucleus,  so  that  we  cannot 
assert  positively  that  any  known  forms  consist  of  un- 
differentiated  protoplasm  as  was  once  supposed  to  be 
*  the  case.  As  a  rule  the 

protoplasm  is  segregated  in 
masses  of  definite  form,  usu- 
ally furnished  with  a  more 
or  less  evident  envelope  and 
provided  with  a  special 
structure,  the  nucleus. 
These  nucleated  masses  of 
protoplasm  are  generally 
called  cells,  although  the 
name  is  occasionally  re- 
served for  such  as  are  pro- 
vided with  a  single  nucleus 
and  a  definite  membrane, 
each  nucleus  with  its  ac- 
companying protoplasm  be- 
ing designated  an  "energid." 
We  shall,  however,  for  con- 
venience' sake  use  the  term 
cell  in  its  ordinarily  accepted 


n  •••• 
nu- 


IDC. 


FIG.  1.  —  A,  a  cell  from  a  hair  at  the 
base  of  the  stamen  of  a  spider- 
wort  (Tradescantia),  showing 
the  parts  of  a  typical  plant  cell; 
w,  the  cell-wall;  pr,  the  proto- 
plasm in  which  is  imbedded  the 
nucleus,  n,  with  the  nucleolus, 
nu;  v,  vacuoles,  spaces  filled 
with  watery  cell-sap.  B,  a  Des- 
mid  (Cosmarium),  a  plant  con- 
sisting of  a  single  cell ;  pi,  one 
of  the  chloroplastids ;  n,  the 
nucleus. 


sense. 

In  all  but  the  lowest  forms  of  life  the  cells  always  show 
at  least  two  parts,  the  cell-plasm  or  cytoplasm,  and 'the 
nucleus.  The  latter  is  usually  of  definite  form,  globular 
or  lenticular  (Fig.  1,  A,  w),  and  bounded  by  a  definite 
membrane,  which,  however,  is  apparently  not  chemically 
different  from  the  cytoplasm  in  which  it  is  imbedded. 


INTRODUCTION  5 

The  nucleus  shows  a  complicated  structure,  being  com- 
posed of  a  very  much  twisted  filament,  more  or  less 
fused  together  at  certain  points,  of  a  substance  (linin) 
which  does  not  readily  take  up  the  ordinary  stains  used 
in  histological  studies.  In  this  linin-thread  are  numer- 
ous granules  of  a  peculiar 
substance,  chromatin,  char- 
acterized by  its  avidity  for 
various  nuclear  stains.  A 
colorless  fluid,  or  semi-fluid 
substance  fills  the  space  of 
the  nuclear  cavity  not  occu- 

•;  FIG.  2.  —  An  Amoeba  —  a  unicel- 

pied     by     the     linin-thread.  lular  organism  consisting  of  a 

„,                    .  nucleated  mass  of  protoplasm 

1  here  are  111  most  Cases  One  without  a  cell-wall,    n,  nucleus; 

or    more    nucleoli   present,  ' 


globular  bodies  which  gen- 

erally stain  strongly,  but  whose  nature  is  still  some- 
what doubtful.  Of  the  various  constituents  of  the 
nucleus,  the  chromatin  is  probably  the  most  important, 
and  it  is  likely  that  in  this  substance  are  contained  the 
elements  which  determine  the  peculiar  properties  of 
the  cell,  and  in  the  reproductive  cells  transmit  heredi- 
tary characters.  The  nucleus  is  an  essential  part  of 
the  cell,  and  is  always  formed  by  division  of  a  preex- 
isting nucleus.  There  is  no  evidence  that  it  can  ever 
arise  de  novo. 

Nuclear  division  is  of  two  kinds,  direct  and  indirect. 
By  the  first  method  a  nucleus  simply  becomes  con- 
stricted and  forms  two  similar  nuclei,  or  the  part  sep- 
arated may  be  smaller  than  the  main  part  of  the  nucleus. 
This  form  of  division  is  confined  to  the  lower  types  of 
plants,  or  it  may  occur  secondarily  in  old  cells  of  some 


6 


EVOLUTION   OF   PLANTS 


of  the  higher  forms,  such  as  the  long  cells  in  the  stems 
of  a  good  many  flowering  plants. 

Much  more  commonly  the  division  of  the  nucleus  is 
preceded  by  a  number  of  complicated  changes,  result- 
ing in  the  breaking 
c  up  of  the  linin-thread 

into  separate  pieces 
or  segments  (chro- 
mosomes) and  a  fur- 
ther splitting  of  these 
segments  into  halves. 
Two  groups  of 
segments  are  thus 
formed,  which  sep- 
arate and  rearrange 
themselves  to  form 
the  daughter-nuclei. 
This  indirect  division 
(Mitosis,  Karyokine- 
sis)  is  the  only  form 
found  in  the  actively 
dividing  cells  of  the 
higher  plants. 

Besides  the  nucleus 
there    are    found    in 

most  plant  cells  certain  bodies  known  as  "plastids." 
(Fig.  1,  B,^>£.)  These  are  similar  to  the  cytoplasm  in 
composition,  and  are  very  important  in  the  nutrition  of 
the  cell.  Among  them  are  the  green  corpuscles  — 
"  chloroplastids  "  or  chromatophores  —  in  which  are 
contained  the  green  pigment,  chlorophyll,  which  plays 
so  important  a  role  in  the  green  plants.  The  red  and 


FIG.  3.  —  Four  cells  from  the  growing  tip  of 
the  root  of  an  onion,  showing  different 
stages  in  the  division  of  the  cell-nucleus. 
In  B  the  nuclear  membrane  has  disap- 
peared and  the  nuclear  segments  or  chro- 
mosomes (cr)  are  arranged  in  a  plate  at 
the  equator  of  the  nuclear  spindle,  which 
is  composed  of  the  "spindle-fibres,"  /. 
In  C  the  two  groups  of  chromosomes 
have  moved  to  the  poles  of  the  nuclear 
spindle.  In  D  the  young  division-wall, 
p,  has  been  formed. 


INTRODUCTION  7 

yellow  corpuscles  found  in  many  flowers  and  fruits  also 
belong  in  this  category. 

Associated  with  the  nucleus  in  most  animal  cells, 
and  sometimes  found  in  those  of  plants,  are  certain 
small  bodies,  the  centrospheres,  each  enclosing  a  minute 
corpuscle,  the  centrosome.  These  are  usually  consid- 
ered to  be  of  great  importance,  especially  in  the  process 
of  nuclear  division,  but  their  absence  from  many  plant 
cells  would  indicate  that  their  importance  has  been  over- 
rated. 

In  the  young  plant  cell  the  cytoplasm  nearly  or  quite 
fills  the  cell,  but  as  the  latter  enlarges  there  is  an  ac- 
cumulation of  fluid  in  the  cell,  and  this  occupies  the 
greater  part  of  its  bulk.  This  watery  cell-sap  is  con- 
tained in  cavities  (vacuoles)  which  it  has  been  claimed  are 
integral  parts  of  the  cell,  and  multiply  by  division ;  but 
this  view  is  by  no  means  universally  admitted. 

New  cells  may  arise  in  several  ways.  The  com- 
monest is  by  fission,  or  division  into  two  parts,  usually 
equal.  This  division  is  preceded  by  division  of  the 
nucleus,  after  which  a  cell- wall  is  formed  dividing  the 
cavity  of  the  cell.  (Fig.  3,  D.)  Less  commonly  there 
is  a  repeated  division  of  the  nucleus,  followed  by  a 
simultaneous  division  of  the  protoplasm  into  as  many 
parts  as  there  are  nuclei.  This  "  internal  division  "  is 
most  common  in  the  formation  of  spores  and  other 
reproductive  cells. 

The  study  of  the  cell,  and  especially  the  changes  in 
the  dividing  nucleus,  have  been  the  subjects  of  some  of 
the  most  important  researches  of  recent  years,  and  have 
developed  a  distinct  department  of  biology,  cytology, 
with  results  of  far-reaching  importance.  Nevertheless, 


8  EVOLUTION   OF  PLANTS 

we  are  still  far  from  understanding  the  ultimate  struct- 
ure of  the  cell,  although  many  ingenious  hypotheses 
have  been  formed  to  explain  the  structure  of  the  proto- 
plasm. 

We  have  seen  that  cells  multiply  by  division.  In 
the  lowest  organisms  the  cells  thus  produced  usually 
separate  at  once,  resulting  in  the  formation  of  two  or 
more  individuals  exactly  like  the  parent.  In  case  this 
division  is  repeated  at  short  intervals,  as  happens,  for 
example,  in  the  Bacteria  and  many  infusorians,  the 
result  is  the  production  of  an  enormous  number  of  in- 
dividuals in  a  surprisingly  short  time.  In  all  but  the 
lower  forms  of  life  the  cells  do  not  usually  separate 
after  division,  the  result  being  a  multicellular  organism. 
The  cell-aggregates,  of  which  these  higher  plants  and 
animals  are  composed,  are  known  as  tissues,  and  these 
may  be  combined  to  form  special  organs.  The  cells  of 
growing  parts  of  the  higher  plants  resemble  t.he  simple 
unicellular  forms  in  structure,  but  as  they  grow  older 
they  may  become  extremely  modified  to  fit  them  for 
special  functions. 

If  we  examine  one  of  the  lower  vegetable  forms,  such 
as  a  desmid  (Fig.  1,  B),  we  find  that  the  single  cell  of 
which  the  plant  is  composed  is  at  once  vegetative  and 
reproductive.  Such  a  green  cell  is  capable  of  perform- 
ing all  the  life-functions.  It  can  absorb  water  con- 
taining certain  food  elements  in  solution,  including  the 
oxygen  necessary  for  respiration,  and,  by  virtue  of  the 
chromatophore  containing  chlorophyll,  is  able  in  the  pres- 
ence of  light  to  decompose  carbon  dioxide  and  water, 
and  from  the  oxygen,  hydrogen,  and  carbon  so  separated, 
to  manufacture  the  primitive  carbo-hydrates  necessary 


INTRODUCTION  9 

for  the  growth  of  the  cell.  The  power  to  manufacture 
these  carbon  compounds  is,  so  far  as  is  positively  known, 
confined  to  cells  which  contain  chlorophyll.  Finally, 
by  division  the  cell  gives  rise  to  two  new  ones,  which 
become  at  once  independent  individuals,  each  contain- 
ing a  nucleus  aixd  chromatophores  like  the  parent  cell. 
This  brief  cycle,  feeding,  growth,  and  division,  consti- 
tutes the  whole  life-history  of  many  of  these  lowly 
organisms. 

As  we  compare  these  simple  plants  with  the  more 
perfect  higher  forms,  we  find  a  more  and  more  marked 
specialization  of  parts  fitting  them  for  special  functions. 
Thus  there  is  very  early  shown  a  modification  of  cer- 
tain cells  for  purely  reproductive  purposes.  These 
cells  are  evidently  descendants  of  vegetative  ones,  and 
in  their  earliest  phases  of  development  are  often  indis- 
tinguishable from  the  latter  ;  finally,  however,  they  be- 
come extremely  modified,  and  can  serve  for  reproduction 
only.  In  extreme  cases  this  results  in  the  formation 
of  sexual  cells,  when  two  sorts  of  cells,  male  and 
female,  are  produced,  each  of  which  is  incapable  of 
developing  further  except  as  the  result  of  a  union  of 
the  two. 

An  analogous  differentiation  of  the  vegetative  parts 
of  the  plant  is  seen  as  we  pass  from  the  lower  to  the 
higher  forms.  While  in  the  unicellular  plant  the  same 
cell  serves  to  perform  all  of  the  functions,  in  the  higher 
plants  special  organs  are  developed  for  special  purposes. 
This,  of  course,  reaches  its  maximum  in  the  seed- 
bearing  plants,  or  "  flowering  plants,"  as  they  are  more 
commonly  known.  Here  not  only  is  the  plant  body 
multicellular,  but  the  cells  show  great  variety  of  form 


10  EVOLUTION   OF   PLANTS 

and  structure,  and  constitute  tissues  of  different  kinds 
which  are  in  turn  aggregated  to  form  special  organs. 
Thus  a  special  subterranean  root  system  is  present,  and 
the  green  assimilative  tissue  is  mainly  confined  to  the 
leaves,  which  are  preeminently  organs  for  carbon  assimi- 
lation. The  extreme  of  specialization  is  reached  in  the 
flowers  of  these  plants,  which  are  beyond  doubt  the 
most  complicated  structures  which  occur  in  the  plant 
kingdom. 

Between  the  extremes  found  in  the  unicellular  plants 
at  the  bottom  of  the  series,  and  the  complicated  seed 
plants  at  the  top,  are  numberless  gradations  of  structure 
which  throw  much  light  upon  how  these  advances  in 
structure  have  been  brought  about.  A  similar  progress 
from  the  simple  to  the  complex  is,  of  course,  evident  in 
the  evolution  of  the  animal  kingdom  ;  but  the  animal 
type  reaches  a  far  greater  degree  of  complexity  and 
specialization  than  is  ever  found  even  in  the  highest 
plants,  which  differ  much  less  from  the  lower  ones  than 
is  the  case  among  animals. 

At  the  bottom  of  the  scale  the  two  kingdoms  con- 
verge. There  are  many  forms  to  be  met  with  whose 
position  is  more  or  less  doubtful,  and  in  some  cases  it 
is  practically  impossible  to  determine  to  which  great 
division  they  belong.  We  can  only  say  that  we  have 
to  do  with  organisms  which  are  not  yet  sufficiently 
differentiated  to  determine  whether  the  animal  or 
vegetable  characters  predominate.  It  is  the  study  of 
these  primitive  organisms,  and  the  realization  of  the 
close  similarity  in  the  structure  and  functions  of  the 
animal  and  plant  cell,  which  emphasize  the  intimate 
connection  between  the  two  kingdoms,  and  the  impos- 


INTRODUCTION  1 1 

sibility  of  making  any  absolute  separation  between 
them. 

A  popular  misconception  of  the  province  of  biology 
assigns  to  it  only  a  study  of  animal  functions  ;  but  the 
scientific  biologist  recognizes  the  fundamental  likeness 
in  the  structure  and  functions  of  plants  and  animals, 
and  realizes  that  any  complete  survey  of  the  science 
must  take  equal  cognizance  of  both. 

For  practical  purposes,  inasmuch  as  all  but  the  lowest 
forms  of  life  are  readily  to  be  assigned  to  one  kingdom 
or  the  other,  it  is  desirable  to  retain  the  old  divisions 
of  zoology  and  botany ;  but  this  does  not  imply  any 
absolute  differences  between  the  two  great  divisions  of 
living  things.  The  popular  belief  that  plants  and  ani- 
mals differ  essentially  in  their  life-processes  is  errone- 
ous. Plants  feed,  breathe,  and  reproduce,  exactly  as  do 
animals.  It  is  true  that  the  green  cells  of  plants  are 
able  to  absorb  carbon  dioxide  from  the  atmosphere,  and 
to  utilize  it  in  the  manufacture  of  carbon  compounds, 
a  power,  which,  so  far  as  we  know,  is  lacking  in  ani- 
mals. This  assimilation  —  or  as  it  has  been  better 
termed  "  photo-synthesis  "  —  is  not  to  be  confounded 
with  respiration,  which  takes  place  in  all  plants  pre- 
cisely as  in  animals,  but,  being  less  energetic,  is 
masked  by  the  evolution  of  an  excess  of  oxygen  in 
those  green  cells  which  are  exposed  to  sunlight.  This 
photo-synthesis,  and  the  character  of  the  cell-wall, 
which  in  young  plant  cells  is  always  composed  of  the 
carbo-hydrate  cellulose,  are  the  most  marked  charac- 
teristics of  ordinary  plants  ;  but  as  cellulose  occurs  in 
some  animals,  i.e.  certain  Tunicates,  and  very  many 
plants  like  the  Fungi,  and  many  parasites  and  sapro- 


12  EVOLUTION   OF   PLANTS 

phytes  among  the  higher  plants,  such  as  the  common 
dodder  and  Indian  pipe,  are  quite  destitute  of  chloro- 
phyll, and  hence  incapable  of  carbon  assimilation,  it 
is  clear  that  neither  of  these  criteria  can  be  used  as 
absolutely  decisive.  Nevertheless,  the  power  of  car- 
bon assimilation,  with  the  accompanying  presence  of 
chlorophyll,  and  the  cellulose  cell-membrane,  are  char- 
acters constant  in  all  typical  plants. 

As  already  indicated,  it  is  near  the  bottom  of  the 
two  great  series  of  organisms  that  they  approach,  and 
as  these  ascending  lines  of  development  are  traced  they 
diverge  widely,  the  peculiar  animal  and  plant  charac- 
ters becoming  more  and  more  pronounced  as  we  as- 
cend. That  special  department  of  biology,  known  as 
Taxonomy  or  Classification,  is  the  attempt  to  group  all 
these  divergent  forms  of  life  so  as  to  indicate  their 
relationships. 

So  long  as  plants  were  considered  as  so  many  isolated 
objects  without  any  genetic  connection,  the  earlier 
systematists,  especially  Linne,  sought  simply  for  some 
obvious  external  characters  which  would  serve  for  iden- 
tification, without  any  thought  as  to  any  real  relation- 
ship. Later  botanists,  although  they  did  not  assume 
any  genetic  relationship,  nevertheless  in  the  so-called 
natural  system,  did  make  an  attempt  to  arrange  them 
in  a  sequence  which  seemed  to  imply  some  such 
connection,  and,  in  many  instances,  really  succeeded, 
although,  through  the  selection,  in  many  instances,  of 
characters  of  secondary  importance,  many  mistakes 
were  made. 

An  ideal  system  of  classification  of  plants  would 
show  the  genealogy  of  the  whole  vegetable  kingdom 


INTRODUCTION  13 

in  all  its  numberless  ramifications.  Such  a  complete 
classification  can  never  be  hoped  for,  inasmuch  as  the 
plants  which  now  exist  are  in  many  cases  but  scattered 
remnants  of  groups  once  much  more  numerous  than 
at  present,  which  have  left  no  recognizable  fossil  traces. 
Some  forms  are  so  much  isolated,  and  have  so  little  in 
common  with  other  groups,  that  at  present  any  attempt 
to  give  them  their  proper  place  in  the  system  is  little 
better  than  pure  guesswork. 

It  is  thus  clear  that  at  present  the  question  is  very 
far  from  settled ;  indeed,  hardly  more  than  a  beginning 
has  been  made  in  the  establishment  of  a  system  which 
can  be  said  to  represent  real  genetic  relationships.  Our 
present  knowledge  of  the  vast  majority,  even  of  many 
of  the  commoner  plants,  is  extremely  imperfect,  being 
confined  often  to  purely  superficial  characters.  It  is 
necessary  to  investigate  thoroughly  the  structure  and 
development  of  a  great  many  forms  before  the  data 
can  be  had  for  constructing  a  classification  which  we 
can  hope  will  be  permanent,  and  a  beginning  only  of 
this  vast  work  has  yet  been  made.  In  addition  to  the 
careful  structural  study  of  the  existing  plants,  a  thor- 
ough examination  of  the  fossil  species  is  necessary, 
involving  even  more  arduous  labor  than  does  the  in- 
vestigation of  living  forms.  Palseo-botany  has  already 
yielded  results  of  the  greatest  importance,  and  it  is  but 
reasonable  to  hope  that  further  investigations  will  add 
much  to  the  materials  already  accumulated.  These 
researches  must,  however,  consist  of  something  more 
than  mere  collecting  and  naming  of  dubious  fragments. 
What  is  imperative  is  a  more  complete  knowledge 
of  the  remains  already  discovered,  rather  than  the 


14  EVOLUTION   OF   PLANTS 

accumulation    of    doubtful    and    imperfectly    studied 
forms. 

While  it  is  true  that  the  great  majority  of  the  fossil 
remains  of  plants  are  too  imperfect  to  make  possible  a 
satisfactory  study  of  their  liner  structure,  it  happens 
occasionally  that  the  tissues  are  preserved  with  extraor- 
dinary completeness,  so  that  a  microscopic  study  of 
the  cellular  structure  is  possible,  and  in  this  way  much 
light  has  been  thrown  upon  the  real  nature  of  many 
fossil  plants.  A  few  types,  like  the  Diatoms,  have 
silicified  cell-walls  which  have  remained  unaffected  by 
the  changes  to  which  they  have  been  subjected,  and 
have  sometimes  been  preserved  in  immense  quantities, 
and  so  perfectly  that  even  the  species  can  be  determined 
without  difficulty.  Other  forms,  with  calcined  cell- 
membranes,  like  the  Coralline  algae  and  Characese,  have 
also  been  preserved  very  perfectly.  In  the  vascular 
plants  the  preservation  of  the  cell-structure  is  usually 
due  to  the  infiltration  of  silicious  matter  after  the 
death  of  the  plant.  These  silicified  tissues,  such  as 
the  familiar  fossil  woods,  often  show  the  cell-structure 
with  marvellous  clearness,  but  unfortunately  the  more 
perishable  tissues  are  very  seldom  preserved  in  this 
way,  and  these  are  usually  of  especial  importance  in 
classification.  Thus  the  flowers  of  the  seed  plants,  and 
the  spore-bearing  parts  of  the  lower  ones,  are  seldom 
preserved  in  a  recognizable  state,  and  this  makes  a 
careful  study  of  such  few  forms  as  have  survived, 
doubly  important,  as  these  are  the  surest  means  of  de- 
ciding the  relationships  of  these  fossil  plants  to  each 
other  and  to  their  living  descendants.  At  best,  the 
geological  record  is  extremely  fragmentary,  and  we 


INTKODUCTION  15 

must  therefore  look  to  other  sources  of  information  in 
our  quest  for  the  ancestors  of  the  present  flora  of  the 
earth. 

Much  can  be  learned  as  to  the  relationships  of  plants 
from  a  study  of  their  external  structure,  and  the  classi- 
fication, especially  of  the  higher  plants,  is  based  largely 
upon  purely  external  characters.  While  such  charac- 
ters are  usually  reliable  when  dealing  with  nearly  re- 
lated forms,  they  are  likely  to  be  misleading  when  we 
try  to  trace  out  the  affinities  of  plants  whose  kinship 
is  not  so  obvious.  Here  it  is  important  to  take  into 
account,  for  comparison,  the  more  obscure  points  of 
structure,  —  for  it  not  infrequently  happens  that  re- 
semblances may  thus  be  traced  which  are  not  evident 
at  first  sight.  Thus,  in  comparing  the  Mosses  and 
Ferns,  it  is  the  minute  reproductive  organs  and  em- 
bryos which  show  the  unmistakable  relationship  of 
these  plants,  while  their  more  conspicuous  external 
structures  are  very  different. 

There  is  little  question  that,  as  in  the  study  of  animal 
forms,  it  is  the  careful  investigation  of  the  life-history 
of  the  plant  which  affords  the  surest  clue  to  its  affinities 
with  other  forms.  The  generally  accepted  view  that 
in  animals  the  developing  germ  repeats  in  a  general 
way  the  evolution  of  the  race,  is  also  applicable,  in 
some  degree  at  least,  to  plants,  and  by  far  the  most 
important  discoveries,  with  reference  to  the  origin  of 
plant  forms,  have  been  due  to  studies  of  this  nature. 
Very  often  the  early  stages  of  the  embryo  and  repro- 
ductive organs  in  different  plants  reveal  resemblances, 
while  the  adult  stages  may  have,  apparently,  very  little 
in  common.  These  embryonic  phases  are  less  affected 


16  EVOLUTION  OF   PLANTS 

by  external  influences,  and  as  they  represent  presuma- 
bly primitive  conditions,  the  importance  of  a  study  of 
these  early  stages  of  the  plant's  existence  is  evident. 

When  we  consider  the  manifold  sources  of  error,  it 
is  not  to  be  wondered  at  that  botanists  have  not  yet 
been  able  to  establish  a  perfect  system  of  classification, 
and  that  it  must  be  a  long  time  before  anything  ap- 
proaching this  can  be  hoped  for. 

The  plant  kingdom  is  usually  divided  into  a  num- 
ber of  primary  divisions,  "branches,"  or  "sub-king- 
doms," as  to  whose  limits  there  is  not  complete  accord 
among  botanists.  Excluding  a  number  of  groups  of 
doubtful  affinity,  sometimes  put  together  under  the 
name  Protophyta,  botanists  usually  recognize  the  fol- 
lowing sub-kingdoms :  1.  Algae  (green  plants  below 
the  Mosses);  2.  Fungi  (a  group  parallel  with  the 
Algie,  but  destitute  of  chlorophyll);  3.  Archegoniatae 
(Mosses  and  Ferns);  4.  Spermatophyta  (seed-plants— 
the  "flowering  plants"  of  the  older  botanists).  Of 
these  divisions,  the  Fungi  and  Algye  are  often  united 
into  a  single  great  division,  Thallophyta,  and  the  Arche- 
goniatre  divided  into  two  sub-kingdoms,  Masses  (Bry- 
ophyta)  and  Ferns  (Pteridophyta) ;  but  the  arrangement 
here  given  seems  to  the  writer  more  in  accordance  with 
what  we  know  of  the  relationships  of  the  different 
members  of  these  groups. 


CHAPTER   II 

THE   CONDITIONS   OF   PLANT   LIFE 

No  matter  how  simple  or  how  complicated  they  may 
be,  all  plants  agree  in  their  essential  life-processes,  and 
certain  conditions  are  necessary  for  these.  All  feed, 
grow,  and  reproduce,  and  all  exhibit  to  a  greater  or 
less  degree  the  power  of  movement,  although  this  is, 
as  a  rule,  much  less  evident  than  in  animals.  For  the 
manifestation  of  the  various  functions,  certain  external 
conditions  are  essential.  Thus  a  certain  amount  of 
moisture  is  necessary  in  order  that  they  may  grow, 
and  of  course  the  requisite  food  elements  must  be  sup- 
plied. In  green  plants,  where  alone,  as  we  have  seen, 
the  assimilation  of  carbon  dioxide  goes  on,  this  is 
dependent  upon  the  presence  of  light,  and  there  are 
certain  limits  of  temperature  also  which  regulate  the 
activity  of  the  plant.  The  amount  of  moisture,  heat, 
and  light  necessary  may  vary  greatly,  however,  in 
different  plants.  Many  water  plants,  especially  algse, 
often  flourish  in  water  whose  temperature  is  very  near 
the  freezing  point,  and  some  of  them  may  be  actually 
frozen  into  the  solid  ice  without  injury ;  but  these 
same  plants  are  quickly  killed  if  they  are  placed,  even 
for  a  short  time,  in  warm  water.  In  strong  contrast  to 
these  are  certain  low  plants,  e.g.  species  of  Oscillaria 
and  various  bacteria,  which  thrive  in  hot  springs  im- 
c  17 


18  EVOLUTION   OF   PLANTS 

pregnated  with  sulphur  and  other  mineral  substances 
usually  inimical  to  plant  life. 

When  in  a  dormant  condition,  the  protoplasm  is  able 
to  resist  much  greater  extremes  both  of  heat  and  cold 
than  is  possible  while  it  is  in  an  active  state.  Thus 
seeds,  spores,  and  the  twigs  of  woody  plants  can  en- 
dure without  injury  a  degree  of  cold  which  would  at 
once  kill  the  protoplasm  were  the  cells  in  a  growing 
condition.  On  the  other  hand,  the  same  dormant  parts, 
especially  spores  of  various  kinds,  can  endure  a  com- 
paratively high  temperature  without  injury,  this  being 
especially  marked  in  the  case  of  the  spores  of  certain 
bacteria,  which  can  endure  exposure  for  several  hours 
to  a  temperature  above  the  boiling  point  of  water 
without  being  killed. 

The  amount  of  moisture  necessary  for  plant  growth 
also  varies  extremely.  Water  plants  are  quickly  killed 
by  exposure  to  air  of  ordinary  dryness,  while"  many 
desert  plants,  such  as  cacti,  may  remain  uprooted 
and  exposed  to  the  hot  sun  for  weeks  without  being 
killed.  These  desert  plants  are  provided  with  very 
perfect  means  of  resisting  loss  of  water,  both  by  a  great 
reduction  of  the  evaporating  surface  through  the  partial 
or  complete  suppression  of  leaves,  and  also  by  the  de- 
velopment of  a  thick,  and  almost  impervious  covering 
to  the  exposed  surfaces. 

In  all  green  plants  the  arrangement  of  the  chloro- 
phyllous  tissue  is  always  regulated  by  the  amount  of 
light.  If  this  is  weak,  the  green  cells  are  spread  out 
so  as  to  expose  a  large  area  to  its  action  ;  but  if  the 
light  is  too  intense  the  area  is  reduced,  and  the  cells 
are  screened  by  the  development  of  more  or  less  opaque 


THE   CONDITIONS   OF   PLANT   LIFE  19 

tissue  above  them.  Plants  growing  in  deep  shade  have 
usually  larger  and  more  delicate  leaves  than  those  fully 
exposed  to  the  sun. 

The  principal  sources  of  plant  food  are  carbon  dioxide 
and  oxygen,  obtained  from  the  atmosphere,  and  water 
with  various  inorganic  substances  in  solution,  usually 
absorbed  by  the  higher  plants  from  the  earth.  When 
the  plant  is  completely  submerged,  as  are  many  algae, 
and  a  considerable  number  of  flowering  plants  also,  the 
food  substances  dissolved  in  the  water  may  be  taken  in 
at  almost  any  point.  Except  in  a  few  doubtful  cases 
among  the  lowest  plants,  all  food  taken  in  must  be  in 
a  gaseous  or  fluid  form. 

Where  the  plant  is  unicellular,  of  course  this  single 
green  cell  must  perform  all  the  nutritive  functions,  and 
is  at  the  same  time  reproductive.  Such  a  simple  plant 
consists  of  a  single  globular  or  oval  cell  surrounded  by 
a  membrane  of  cellulose,  within  which  is  the  nucleated 
protoplasmic  mass  with  one  or  more  chromatophores 
or  chloroplasts.  Such  a  cell  can  absorb  water  with 
various  food  substances,  including  free  oxygen  and 
carbon  dioxide  in  solution.  The  chromatophores,  in 
some  way  not  clearly  understood,  decompose  the  car- 
bon dioxide  and  water,  and  of  the  elements  carbon, 
hydrogen,  and  oxygen,  manufacture  the  carbo-hydrates 
upon  which  the  protoplasm  is  dependent  for  its  growth. 
The  first  product  of  this  process  which  can  be  recog- 
nized, is  usually  starch,  which  appears  in  the  form  of 
granules  within  the  chloroplasts  shortly  after  they  are 
exposed  to  the  action  of  light,  which,  as  we  have  seen, 
is  a  necessary  condition  for  photo-synthesis. 

As  a  result  of  the  assimilation  of  the  food  absorbed, 


20  EVOLUTION   OF   PLANTS 

the  cell  increases  in  bulk,  and  sooner  or  later  divides 
to  form  new  ones  which  become  at  once  new  individuals, 
so  that  the  single  cell  exhibits  all  the  characteristics  of 
a  typical  plant;  i.e.  it  feeds,  grows,  and  reproduces 
itself,  and  all  these  functions  are  performed  by  one 
and  the  same  cell.  Such  a  unicellular  plant  cannot 
properly  be  considered  as  strictly  undifferentiated, 
since  the  permanent  constituents  of  the  cell,  e.g. 
nucleus  and  chromatophore,  must  be  regarded  as  defi- 
nite organs. 

As  we  pass  from  the  unicellular  plants  to  the  simpler 
multicellular  forms  (see  Fig.  9,  F)  we  find  the  first 
indications  of  a  specialization  of  certain  cells.  Thus  the 
basal  cell  has  very  little  chlorophyll,  but  is  modified 
into  a  root-like  organ  for  the  attachment  of  the  plant, 
while  the  other  cells  with  their  numerous  chloroplasts 
are  alone  concerned  with  the  nutrition  of  the  plant. 
Very  much  more  complicated  are  many  of  the  large 
sea-weeds,  some  of  which,  like  the  great  kelps  (see 
Fig.  17),  reach  an  enormous  size.  In  these  plants 
there  are  various  sorts  of  cells  aggregated  into  definite 
organs.  The  chloroplasts  are  mainly  confined  to  the 
outer  part  of  the  plant,  where  they  may  be  fully  exposed 
to  the  light,  while  the  inner  tissue  has  little  or  no 
chlorophyll,  and  the  cells  are  modified  for  conducting 
purposes.  In  the  most  highly  organized  of  these  marine 
algae,  like  the  gulf-weed  (Fig.  18),  a  further  advance 
is  seen  in  the  formation  of  flattened  leaves  to  which 
the  chlorophyll-bearing  cells  are  mainly  restricted. 
Indeed  these  nighty  specialized  sea-weeds  bear  a  most 
remarkable  superficial  resemblance  to  the  flowering 
plants  in  the  development  of  a  definite  branching  axis 


THE   CONDITIONS   OF   PLANT   LIFE  21 

bearing  leaves.  This  is  one  of  many  instances  where 
in  response  to  similar  needs  there  has  been  a  parallel 
development  in  groups  which  genetically  are  widely 
separated. 

It  is,  of  course,  among  the  Spermatophytes  (seed- 
bearing  or  flowering  plants)  that  the  highest  degree 
of  specialization,  both  of  the  plant  body  and  tissues,  is 
reached.  The  plant  usually  shows  a  definite  main  axis 
or  stem,  to  which  are  attached  a  variety  of  appendicular 
organs  —  leaves,  roots,  and  branches.  The  tissues  of 
which  these  various  organs  are  composed  show  much 
variation  in  the  cells  of  which  they  are  made  up.  The 
green  tissue  is  mainly  restricted  to  the  leaves,  where 
it  is  so  placed  as  to  be  most  favorably  situated 
with  reference  to  the  light.  As  a  rule  the  outer  or 
epidermal  cells  are  not  provided  with  chloroplasts,  but 
serve  as  a  protection  for  the  delicate  green  cells  lying 
below  them,  and  in  case  the  plant  is  exposed  to  great 
heat  or  dryness,  the  epidermal  cells  become  much 
thickened  and  almost  impervious  to  water,  so  that  the 
loss  of  water  from  the  green  cells  is  effectively  checked. 
Familiar  examples  of  this  kind  are  seen  in  the  leaves 
of  the  laurel,  oleander,  and  many  other  evergreens. 
As  it  is  necessary,  however,  for  the  green  cells  to  have 
communication  with  the  atmosphere  in  order  to  obtain 
the  necessary  carbon  dioxide  and  oxygen,  this  is  pro- 
vided for  by  the  development  of  the  stomata  or  breath- 
ing pores  always  found  upon  the  leaves,  and  these 
communicate  with  the  numerous  air-spaces  between 
the  green  cells  which  are  thus  brought  directly  into 
contact  with  the  atmospheric  gases. 

Within  the  green  cells  the  decomposition  of  the  car- 


22  EVOLUTION   OF   PLANTS 

bon  dioxide  and  water  is  accomplished,  and  from  their 
elements  are  manufactured  the  organic  carbon  com- 
pounds. When  these  green  cells  are  exposed  to  the 
light,  starch  can  soon  be  detected  in  them,  but  it  dis- 
appears if  the  plant  is  placed  for  a  short  time  in  dark- 
ness. 

The  presence  of  other  pigments,  such  as  the  red  and 
yellow  ones  in  the  marine  algse,  and  also  the  similar 
ones  often  found  in  young  leaves,  doubtless  advan- 
tageously modify  the  light  which  passes  through  them 
before  reaching  the  chlorophyll. 

Movement  is  not  generally  associated  with  one's  idea 
of  a  plant,  but  it  is  a  property  which  all  plants  possess 
to  some  degree,  and  is  usually  associated  with  the 
sensitiveness  of  living  protoplasm  within  the  cells. 
In  every  living  cell  the  protoplasm  shows  more  or 
less  marked  movements  which  may  not  be  at  once 
perceptible,  but  sometimes  are  very  active  indeed. 
These  movements  are  very  familiar  to  botanists  in  the 
cells  of  many  water  plants,  e.g.  the  eel-grass  (Vallis- 
neria)  and  stone-wort  (Chara),  and  are  also  very  active 
in  the  cells  forming  the  hairs  upon  the  surface  of  many 
land  plants.  This  is  especially  true  of  the  hairs  upon 
various  parts  of  many  flowers. 

Spontaneous  movements  of  the  plant  as  a  whole 
are  confined  to  a  comparatively  small  number  of  low 
aquatic  forms  (see  Fig.  6).  Here  the  plant  moves  by 
means  of  vibratile  protoplasmic  threads  or  cilia,  which 
propel  it  through  the  water,  precisely  as  many  of  the 
lower  animals  move.  The  extraordinary  resemblance 
between  these  low  ciliated  plants  and  the  lower  animals 
is  one  of  the  strongest  evidences  of  the  relationship 


THE   CONDITIONS   OF   PLANT   LIFE  28 

which  exists  between  the  lowest  members  of  the  two 
great  organic  kingdoms.  This  power  of  free  locomotion 
is  also  found  in  some  of  the  reproductive  cells  of  most 
plants  except  the  highest  ones,  and  even  in  a  few  of 
the  seed  plants  these  have  recently  been  discovered, 
the  male  reproductive  cells  having  the  form  of  ciliated 
spermatozoids  which  are  actively  motile.  This  power 
of  spontaneous  locomotion  is  finally  lost,  and  in  nearly 
all  the  seed  plants  is  completely  absent,  although  even 
here  the  plant  exhibits  more  or  less  marked  movements 
of  various  kinds.  Some  of  these  movements  are  appar- 
ently spontaneous,  such  as  the  revolution  of  the  apex 
of  the  growing  stem  and  root,  and  of  tendrils,  but  oth- 
ers are  influenced  directly  by  external  agencies,  light, 
moisture,  contact,  and  gravity.  The  spontaneous  move- 
ments of  the  growing  apex  of  many  plants  is  called 
nutation,  and  is  apparently  quite  independent  of  ex- 
ternal agencies.  The  effect  of  light  in  plants  is  well 
known.  The  response  of  actively  growing  plants  to 
this  stimulus  is  often  very  rapid,  although  the  exact 
mechanism  of  these  movements  is  not  entirely  clear. 
Occasionally  plants  are  negatively  heliotropic,  i.e.  they 
grow  away  from  the  light,  as  is  seen  in  the  common  ivy. 
Light  is,  in  most  cases,  necessary  for  the  formation  of 
chlorophyll,  as  well  as  for  the  performance  of  photo- 
synthesis in  those  cells  which  contain  the  chlorophyll. 
Doubtless  the  cause  of  the  movements  of  plants  is 
largely  due  to  the  direct  effect  of  light  upon  the  sen- 
sitive protoplasm  of  the  cells  of  the  motile  parts. 
This  is  indicated  by  the  activity  of  naked  swarm- 
spores  and  plasmodia  (masses  of  naked  protoplasm 
found  in  certain  low  organisms,  such  as  the  Sliine- 


24  EVOLUTION   OF   PLANTS 

moulds,  Fig.  4),  when  exposed  to  light,  the  green 
swarm-spores  of  algae  almost  always  being  very  quickly 
attracted  to  the  light. 

Geotropism,  or  the  movements  induced  by  gravity, 
are  either  negative  or  positive.  In  the  higher  plants, 
the  aerial  parts,  especially  the  stems,  are  usually  nega- 
tively geotropic,  i.e.  grow  upward,  the  roots  positively 
geotropic. 

The  marked  movements  of  the  sensitive  plant  (Mi- 
mosa), as  well  as  the  similar  movements  especially  of  the 
floral  parts  of  many  plants,  and  the  so-called  "  sleep  move- 
ments "  of  such  leaves  as  the  locust,  clover,  and  many 
others,  are  connected  with  changes  in  the  turgor  of  the 
cells  of  special  parts  of  the  motile  organs.  These 
movements  are  undoubtedly,  like  the  movements  due 
to  simple  heliotropism,  intimately  associated  with  the 
sensitiveness  of  the  protoplasm,  and  are  induced  by  a 
variety  of  stimuli,  such  as  shocks,  light,  and  electricity. 

The  absence  of  locomotion  in  the  higher  plants  is  in 
large  measure  due  to  the  investment  of  the  cells  with  a 
firm  membrane,  and  it  is  only  when  the  protoplasm 
escapes  from  the  cell,  as  in  the  case  of  swarm-spores 
and  spermatozoids,  that  the  primitive  power  of  locomo- 
tion is  regained,  and  recalls  the  possession  of  this 
animal  character  by  the  ancestors  of  all  the  higher 
plants. 

It  is  a  popular  fallacy  that  plants  and  animals  supple- 
ment each  other  in  their  method  of  respiration.  It  is 
not  necessary  to  remind  the  botanist  that  this  mistake 
is  based  upon  a  confusion  of  terms.  During  the  process 
of  carbon  assimilation  in  the  green  cells,  there  is  a  large 
amount  of  free  oxygen  liberated  —  much  in  excess  of 


THE   CONDITIONS   OF   PLANT  LIFE  25 

the  amount  used  by  the  cells  in  respiration  ;  hence 
when  exposed  to  light  the  cells  give  off  an  excess  of 
free  oxygen  which  escapes  into  the  surrounding  atmos- 
phere. So  soon,  however,  as  the  light  ceases  to  act,  it 
is  found  that  these  cells,  like  the  colorless  ones,  con- 
sume free  oxygen,  and  the  oxidation  in  the  protoplasm 
is  accompanied  by  an  evolution  of  heat,  precisely  as  in 
animals,  although,  as  a  rule,  it  is  less  energetic.  Occa- 
sionally this  evolution  of  heat  is  quite  perceptible  and 
can  be  easily  measured.  A  thermometer  thrust  into 
a  mass  of  germinating  seeds,  or  into  the  spathe  of  one 
of  the  larger  Aroids  like  the  common  "  calla-lily,"  will 
show  a  rise  of  several  degrees,  this  evolution  of  heat 
in  the  latter  case  being  most  energetic  while  the  pollen 
is  being  shed. 

The  gradual  evolution  of  the  reproductive  parts  of 
plants  is  very  instructive,  especially  when  we  consider 
the  fact  that  it  closely  parallels  the  development  of 
these  parts  in  animals,  this  being  especially  true  of  the 
sexual  reproductive  elements. 

In  the  lowest  forms  of  life,  both  plant  and  animal, 
the  entire  .unicellular  organism  is  at  once  vegetative 
and  reproductive.  In  these  forms,  after  the  cell  has 
attained  its  maximum  size,  it  divides  directly  into 
two  or  more  parts,  each  of  which  becomes  at  once  an 
individual.  The  power  of  forming  new  individuals 
non-sexually  persists  in  many  multicellular  animals, 
and  in  most  plants.  In  animals  the  power  of  produc- 
ing new  individuals  by  budding  or  fission  is  found  in 
a  considerable  number  of  the  lower  groups  of  Metazoa, 
such  as  the  Corals,  Sea-anemones,  etc.,  and  the  renewal 
of  lost  parts  may  take  place  even  in  Vertebrates,  e.g. 


26  EVOLUTION   OF   PLANTS 

in  lizards  the  renewal  of  the  tail,  but,  owing  to  the 
great  complexity  of  the  higher  animals,  they  are  unable, 
except  in  rare  instances,  to  produce  new  individuals 
except  from  eggs.  With  plants  the  case  is  different, 
and  even  among  the  highest  ones  some  forms  of  non- 
sexual  multiplication  by  budding  is  almost  universal. 
This  is  no  doubt  largely  due  to  the  much  lower  degree 
of  specialization  in  the  tissues  of  even  the  highest  plants. 
The  first  evidence  of  sex  is  manifest  very  early  among 
both  animals  and  plants.  Sexual  reproduction  consists 
essentially  in  the  formation  of  a  germ  by  the  union  of 
two  cells  which  fuse  completely  into  one.  In  many 
unicellular  plants,  such  as  the  desmids  (Fig.  1,  B)  and 
lower  green  monads  (Fig.  6,  F),  there  is  no  apparent 
difference  between  the  sexual  and  non-sexual  cells,  but 
two  individuals  fuse  into  one,  or  at  least  the  protoplasm 
and  nuclei  of  the  two  cells  fuse,  the  resulting  cell  then, 
as  a  rule,  secreting  a  new  wall  about  itself,  and  either 
forming  a  new  plant  at  once,  or  by  division  giving  rise 
to  two  or  more  new  plants.  In  these  lowest  forms  the 
two  uniting  cells  are  entirely  similar,  and  we  cannot 
speak  of  male  and  female  cells. 

/  The  first  indication  of  the  separation  of  the  sexes  is 
(  seen  in  the  formation  of  sexual  cells  or  gametes,  of  un- 
equal size  (Fig.  6,  F).  These  cells  are  usually  motile, 
being  provided  with  cilia,  and  resemble  exactly  the  non- 
sexual  swarm-spores,  except  that  they  are  incapable  of 
germinating  unless  two  of  them  unite  to  form  the 
"zygote,"  or  germ  of  the  new  plant.  The  larger  of 
the  gametes  is  the  female,  the  smaller  the  male  cell.  It 
is  interesting  to  note  that  in  some  of  the  lowest  forms 
where  gametes  occur,  these  may  under  certain  coiidi- 


THE   CONDITIONS   OF  PLANT  LIFE  27 

tions  germinate  without  fusion,  showing  that  they  may 
properly  be  considered  simply  as  modifications  of  cells 
once  purely  non-sexual  in  character. 

As  the  sexual  cells  become  more  differentiated,  the 
difference  in  size  becomes  very  marked,  the  female  cell 
being  many  times  larger  than  the  male  (Fig.  6,  D,  E). 
The  former  also  shows  a  tendency  to  become  passive 
before  fertilization,  even  in  such  forms  as  still  retain 
the  primitive  ciliated  condition,  and  finally  all  power  of 
motion  is  lost,  and  usually  the  female  cell,  or  egg,  is  re- 
tained within  the  cell  where  it  is  formed,  and  is  there 
fertilized  by  the  small,  active  male  cell  or  spermatozoid. 

As  the  plant  body  becomes  multicellular,  the  repro- 
ductive function,  except  in  the  lowest  types,  becomes 
restricted  to  special  cells  which  differ  in  appearance 
and  size  from  the  vegetative  cells.  This  is  accompa- 
nied by  the  more  perfect  differentiation  of  the  sexual 
cells,  resulting  as  already  stated  in  the  formation  of  a 
large,  passive  female  cell  or  egg,  and  a  small,  actively 
motile  male  cell  or  spermatozoid.  In  extreme  cases, 
such  as  the  ferns  and  mosses,  the  spermatozoid  is  mainly 
reduced  to  the  nuclear  substance  of  the  mother-cell,  a 
small  portion  only,  including  the  locomotive  organs 
(cilia),  being  composed  of  the  cytoplasm  or  cell-plasm. 

It  is  an  interesting  fact  that  a  very  similar  evolution 
of  the  sexual  cells  has  taken  place  in  the  animal  king- 
dom, and  has  also  developed  independently  in  several 
widely  separated  groups  of  plants.  Thus  we  have  still 
existing,  every  phase  of  development  of  these  sexual 
cells  in  the  Brown  Algse,  the  Volvocacese,  the  Siphonese, 
and  Confervacese,  and  less  perfectly  in  several  other 
groups. 


28  EVOLUTION  OF  PLANTS 

While  in  the  lower  groups  the  sexual  cells  are  borne 
in  cells  differing  but  little  from  the  vegetative  ones, 
the  higher  algse,  mosses,  and  ferns  have  them  con- 
tained in  multicellular  structures  of  very  characteristic 
form  which  may  properly  be  considered  as  true  sexual 
organs. 

A  marked  degeneration  of  the  sexual  cells  is  observed 
in  many  fungi  where,  when  present  at  all,  they  are 
usually  reduced  in  structure  and  sometimes  apparently 
functionless,  while  in  very  many  of  them  no  traces  of 
sexual  organs  have  as  yet  been  discovered. 

Among  the  flowering  plants  there  are  produced  spe- 
cial accessory  structures  connected  with  reproduction 
but  not  to  be  considered  strictly  themselves  as  repro- 
ductive. The  various  parts  of  the  flower  are  of  this 
nature,  the  true  reproductive  organs  being  special 
minute  structures  within  the  pollen-grain  and  ovule. 
The  development  of  brightly  colored  and  sweet-scented 
flowers  is  doubtless  connected  with  the  fertilization  of 
the  germ-cells  within  the  ovule,  and  the  same  is  true  of 
the  various  mechanical  devices  for  insuring  pollination 
through  insect  agency.  The  correlation  of  structures 
in  flowers  and  insects  is  often  extraordinary,  and  is 
sometimes  so  great  that  a  single  species  of  flower  and 
insect  are  absolutely  dependent  on  each  other  for  their 
existence.  We  shall,  however,  consider  this  question 
more  at  length  in  a  later  chapter. 

A  high  degree  of  specialization  is  also  seen  in  the 
subsidiary  reproductive  parts  of  plants  other  than  the 
seed  plants,  although  less  marked  than  in  those.  Thus 
in  the  mosses  and  ferns  there  are  very  perfect  me- 
chanical devices  for  distributing  the  ripe  spores.  The. 


THE   CONDITIONS   OF  PLANT  LIFE  29 

mechanism  of  these  devices  is  usually  dependent  upon 
moisture.  Of  these  may  be  cited  the  ring  of  thickened 
cells  which  surrounds  the  spore-case  in  most  ferns,  and 
the  strongly  hygroscopic  elaters  of  most  liverworts, 
and  the  curious  structures  forming  the  "  peristome " 
about  the  opening  of  the  spore-bearing  capsule  of  the 
common  mosses. 

The  fruits  and  seeds  of  the  flowering  plants  offer 
numberless  examples  of  specialized  structures,  evidently 
adaptations  to  special  environment.  In  the  lower  mem- 
bers of  the  group,  such  as  the  pond-weeds  and  similar 
simple  aquatic  types,  the  fruits  are  very  simple  and 
the  seeds  are  set  free  through  its  decay,  falling  to  the 
bottom  of  the  water,  where  they  remain  until  conditions 
are  suitable  for  germination.  A  similar  condition  of 
things  prevails  among  a  good  many  land  plants,  e.g. 
some  of  the  grasses,  but  in  very  many  of  the  higher 
types  special  contrivances  have  been  evolved  by  means 
of  which  the  distribution  of  the  seeds  is  facilitated. 
The  violent  opening  of  many  seed  vessels  ;  the  wings 
and  floats  developed  by  many  seeds  and  fruits  ;  the 
hooks,  prickles,  etc.,  found  in  many  fruits  and  seeds, 
by  means  of  which  they  adhere  to  animals,  and  are 
thus  transported,  are  a  few  of  these  many  devices,  and 
the  presence  of  edible  parts  in  fruits  and  seeds  is 
largely  to  be  placed  in  the  same  category. 

SUMMARY 

All  plants  agree  in  requiring  for  their  existence 
certain  food  substances  which  must  be  absorbed  in  the 
form  of  solutions  or  gases.  Of  these  food  substances 


30  EVOLUTION   OF   PLANTS 

the  carbon  is  directly  available  only  for  those  plants 
which  possess  chlorophyll ;  and  where  the  plant  is  desti- 
tute of  this,  as  in  most  saprophytes  and  parasites,  it 
must  obtain  its  carbon  from  organic  compounds.  All 
plants  breathe  by  taking  in  free  oxygen  and  giving  off 
carbon  dioxide. 

Water  is  essential  for  the  manifestation  of  life,  but 
the  amount  varies  greatly  in  different  plants  as  does  the 
temperature  at  which  they  will  grow. 

Light  is  necessary  to  all  green  plants,  and  to  most 
others  as  well,  but  among  these,  the  optimum  illumina- 
tion is  extremely  variable. 

We  see,  in  passing  from  the  simple  unicellular  plants, 
where  all  the  functions  are  performed  by  a  single  cell, 
how  there  is  a  gradual  division  of  labor,  first  in  a 
separation  of  the  vegetative  and  reproductive  cells,  and 
later  a  further  specialization  of  both  vegetative  and 
reproductive  functions,  which  reaches  its  highest  ex- 
pression in  the  seed  plants  where  there  are  special  com- 
plicated organs  —  leaves  —  for  carbon  assimilation,  an 
extensive  system  of  roots  for  attachment  and  absorp- 
tion of  food  from  the  soil,  extremely  modified  tissues 
for  conduction,  storing  of  food,  and  other  functions ; 
and  finally  extraordinarily  varied  structures  —  the 
flowers  —  connected  with  the  reproduction  of  the  plant 
and  the  distribution  of  seeds. 

The  power  of  independent  locomotion  is  confined  to  a 
small  number  of  the  lower  plants,  or  to  certain  repro- 
ductive cells  of  higher  ones  ;  but  all  plants  exhibit  more 
or  less  marked  movements  which  may  be  spontaneous, 
or  influenced  by  various  external  agencies,  such  as  light 
and  gravity. 


CHAPTER   III 

THE   SIMPLEST  FORMS   OF  LIFE 

THE  simplest  conceivable  living  being  is  a  mass  of 
undifferentiated  protoplasm,  and  it  was  claimed  by 
Haeckel  and  others  that  such  simple  forms  of  life 
do  actually  exist.  Haeckel  described  under  the  name 
Monera  a  considerable  number  of  organisms  to 
which  he  attributed  this  simple  structure ;  but  the 
great  improvements  made  of  late  years  in  microscopic 
technique  have  shown  that  these  are  really  much  less 
simple  than  was  supposed.  A  nucleus  can  in  most 
cases  be  demonstrated,  as  well  as  other  evidences  of 
differentiation. 

There  are,  however,  a  good  many  organisms  of  such 
simple  structure  that  we  cannot  positively  assert  that 
they  belong  to  either  the  animal  or  vegetable  kingdoms. 
There  are  in  particular  two  groups  of  these  indifferent 
organisms,  those  included  by  Haeckel  in  his  Monera, 
such  as  Vampyrella  and  Protomyxa,  and  those  curi- 
ous organisms  the  "  Slime-moulds,"  —  Myxomycetes  or 
Mycetozoa.  These  two  groups,  which  are  generally 
considered  respectively  as  the  lowest  of  the  animal  and 
plant  series,  have  a  good  many  characters  in  common. 
They  are  all,  in  their  vegetative  condition,  naked  masses 
of  soft,  slimy  protoplasm  —  "  plasm odia," — which  show 

31 


32 


EVOLUTION  OF   PLANTS 


active  movements  and  other  evidences  of  life  (Fig.  4,  A). 
The  Monera  are  aquatic,  often  parasitic,  organisms  and 
their  life-history  is  a  simple  one.  After  the  plasmo- 
dium has  reached  its  maximum  size  it  contracts  and 
develops  a  firm  covering  and  the  contents  of  this  cyst 

divide  into  a  great  many 
parts,  each  of  which  is  pro- 
vided with  a  nucleus.  These 
minute  nucleated  masses  es- 
cape in  the  form  of  actively 
swimming  bodies  which  may 
form  a  new  plasmodium  by 
simple  growth ;  but  some- 
times, by  the  fusion  of  a 
great  number  of  the  sepa- 

^sC^SIi61l!l£|§^  rate  spores,  as  in  Protomyxa, 
a  large  plasmodium  may  be 
formed  at  once. 

In  the  Mycetozoa,  or  Slime- 
moulds,  which  are  often  re- 
garded as  plants,  the  life- 
cycle  is  somewhat  more 
complicated,  due  to  the  fact, 
perhaps,  that  they  are  terres- 
trial in  their  habits.  The 
active  condition  in  these  is 
also  that  of  a  plasmodium, 
these  naked  masses  of  white  or  yellow  slimy  matter 
often  reaching  a  large  size.  The  commonest  of  these 
is  one  found  growing  upon  old  tan-bark  where  the  light- 
yellow  soft  mass  may  often  be  met  with  in  damp,  cloudy 
weather,  the  plasmodium  shunning  too  strong  lights 


T3C. 

FIG.  4  (Mycetozoa).  —  A,  a  Slime- 
mould  growing  upon  a  bit  of 
rotten  wood  ;  B,  two  of  the 
fruiting  structures  (sporan- 


Trichia;  D,  the  active  mo- 

tile  protoplasmic  mass  which 
escapes  from  the  spore  when 
it  germinates  ;  f,  the  flagellum 
or  motile  organ  ;  n,  the  nu- 


flagellum  and  later  unite 
with  others  to  form  the  larger 
"plasmodium,"  shown  in  A. 
v,  the  contractile  vacuole. 


THE   SIMPLEST   FORMS    OF   LIFE  33 

and  needing  moisture  for  its  growth.  If  threatened 
with  drying  up  it  may  contract  and  secrete  a  protec- 
tive covering  about  itself. 

The  reproduction  is  purely  non-sexual,  and  consists 
in  the  breaking  up  of  the  protoplasm  into  a  great  many 
parts,  as  in  the  Monera  ;  but  in  the  slime-moulds  the 
cells  thus  formed  secrete  a  definite  protective  covering 
or  cell- wall,  and  closely  resemble  the  spores  or  repro- 
ductive cells  of  the  Fungi,  with  which  these  organisms 
are  often  classed  ;  but  the  general  opinion  at  present 
is  that  they  are  forms  allied  to  the  Monera,  which  have 
not  yet  become  sufficiently  differentiated  to  show  defi- 
nite animal  or  plant  characters. 

The  most  specialized  forms  among  the  slime-moulds, 
like  the  ones  figured  (Fig.  4),  show  a  curious  resem- 
blance to  true  plants  in  their  reproductive  parts, 
although  these  resemblances  are  purely  superficial. 
Thus  they  form  spore-cases  or  "  sporangia  "  of  definite 
and  characteristic  shapes  (B),  within  which  the  proto- 
plasm divides  into  a  great  many  nucleated  fragments, 
as  in  the  Monera  ;  but  here,  as  we  have  seen,  each  por- 
tion secretes  a  definite  cell-membrane  and  forms  a 
spore,  much  like  an  ordinary  plant-cell  (Fig.  4,  C), 
and  capable  of  being  dried  up  without  losing  its  vital- 
ity. With  these  spores  are  found  in  the  higher  forms 
curious  thread-like  structures  of  various  kinds. 

The  spores,  on  being  placed  in  water,  soon  burst  open 
and  set  free  the  contained  protoplasm,  which  assumes 
the  form  of  a  free-swimming,  naked  cell  or  swarm- 
spore,  like  that  of  the  Monera,  and  resembling  closely 
certain  low  animal  forms,  the  flagellate  infusorians 
(Fig.  4,  D).  In  this  condition  the  slime-mould  con- 

^^^^ 

^  "       O*     f** 

"UNIVERSITY 


34  EVOLUTION  OF  PLANTS 

tinues  for  a  longer  or  shorter  time,  and  may  multiply 
by  fission  and  thus  produce  a  large  number  of  indi- 
viduals which  finally  lose  the  single  cilium  or  flagellum, 
and  creep  about  like  amoebae  (E).  Finally,  as  in  the 
higher  Monera,  these  individuals  fuse  into  a  single  large 
mass  or  plasmodium. 

A  study  of  these  two  groups,  Monera  and  Myceto- 
zoa,  illustrates  in  a  very  instructive  way  how  a  consid- 
erable degree  of  differentiation  is  possible  within  the 
limits  of  a  group  whose  structure  is  of  the  simplest 
character.  The  two  classes  are  probably  offshoots  of 
a  common  stock  very  near  the  bottom  of  the  scale  of 
living  organisms.  It  is  not  likely  that  either  class  has 
much  in  common  with  the  higher  plants  or  animals, 
but  the  constant  occurrence  in  both  of  flagellate  swarm - 
spores  indicates  that  the  latter  may,  perhaps,  represent 
the  simplest  expression  of  living  things  known  to  us, 
and  that  from  some  such  forms  have  sprung  not  only 
the  Monera  and  Mycetozoa,  but  also  the  higher  animals 
and  plants. 

SCHIZOPHYTA  (Fission  Plants) 

Under  this  name  are  now  united  a  large  number  of 
plants  of  very  simple  organization,  of  which  the  most 
familiar  are  the  Bacteria.  Owing  to  the  extreme 
minuteness  of  many  of  them,  it  is  not  possible  to  deter- 
mine positively  how  far  their  apparently  excessive 
simplicity  is  real.  With  better  methods  of  fixing  and 
staining,  and  improved  microscopic  lenses,  the  bacteria 
are  revealing  structures  which  formerly  escaped  detec- 
tion, and  it  is  reasonable  to  suppose  that  there  is  still 
much  to  be  learned  as  to  their  minute  structure. 


THE   SIMPLEST   FORMS   OF   LIFE 


35 


Most  bacteria  appear,  under  the  microscope,  as  ex- 
tremely small,  often  apparently  homogeneous  bodies 
of  various  shapes  —  round,  oblong,  rod-shaped,  etc. 
(Fig.  5).  They  frequently  exhibit  active  move- 
ments which  are  due  to  the  presence  of  excessively 
fine  cilia.  They  multiply  with  extraordinary  rapidity 


3C. 


FIG.  5  (Schizophy  ta) . —  A,  the  tip  of  a  filament  of  Oscillaria,  one  of  the 
Fission  Algae  (Schizophycese).  The  cell  is  filled  with  granular  proto- 
plasm, but  no  definite  nucleus  or  plastids  can  be  made  out.  B,  part  of 
a  filament  of  Anabeena,  a  fission  alga,  showing  two  sorts  of  cells; 
x,  one  of  the  "  heterocysts  "  which  separate  the  filament  into  segments ; 
C,  a  water-plant  with  colonies  of  a  fission  alga  (Gloeotrichia) ,  gl,  grow- 
ing upon  it;  I),  Beggiatoa,  a  form  without  chlorophyll,  allied  to  Oscil- 
laria; E-H,  different  forms  of  Bacteria  (Schizomycetes) ;  E,  typhus 
germ  (Bacillus  typhi) ;  F,  Tetanus  bacillus  (B.  tetani),  showing  spore 
formation ;  G,  cholera  bacillus  (Microspira  comma) ;  H,  spirillum  ru- 
brum ;  G  and  H  are  stained  to  show  the  cilia.  (Figs.  D-H  after  Migula.) 

by  transverse  fission,  but  may  also  produce  internally 
special  resting-cells,  or  spores  (F).  These  latter 
are  thick  walled,  and  often  capable  of  enduring  an 
astonishing  degree  of  heat  without  injury.  Organic 
decomposition  is  mainly  due  to  the  activity  of  bacteria, 
and  it  is  unnecessary  to  dwell  upon  the  various  forms 


36  EVOLUTION   OF   PLANTS 

of  disease  germs,  nearly  all  of  which  are  bacteria.  The 
great  importance  of  these  minute  organisms  in  the 
economy  of  nature  is  at  once  evident  when,  we  reflect 
that,  without  their  assistance,  the  decomposition  of  dead 
organic  matter  would  practically  cease,  and  it  would 
remain  inert  and  useless  as  food  for  the  higher  plants. 
The  presence  of  bacteria  in  the  soil  is  of  the  greatest 
importance,  as  it  is  through  their  agency  that  the  nitro- 
gen compounds  are  put  in  such  form  that  they  can  be 
absorbed  by  the  roots  of  the  higher  plants. 

While  the  position  of  the  bacteria  is  unquestionably 
very  low  in  the  scale,  their  relation  to  the  higher  plants 
is  somewhat  problematical.  The  presence  of  cilia  has 
suggested  a  possible  connection  with  the  flagellate 
infusorians.  Related  to  them  is  a  peculiar  group  of 
simple,  green  plants  known  variously  as  "  CyanophyceaB  " 
—  Blue-green  Algee  —  or  "  Schizophycese  "  -  Fission 
Algse.  Like  the  bacteria  they  multiply  ordinarily  by 
simple,  transverse  fission,  but  may  also  produce  resting- 
spores.  Being  provided  with  chlorophyll,  however, 
they  are  to  some  extent  independent,  but  they  often 
occur  in  such  positions  as  to  indicate  a  partial  depend- 
ence on  other  plants  for  food.  Some  occur  regularly 
within  the  bodies  of  higher  plants,  and  are  probably 
parasitic  to  a  limited  extent.  More  commonly  they  live 
free  upon  damp  earth,  or  in  stagnant  water  (Fig.  4, 
A,  B,  C). 

Like  the  bacteria,  the  cell-structure  is  very  simple, 
and  it  is  doubtful  whether  a  perfectly  organized  nucleus 
is  ever  present,  although  a  central  structure  of  doubtful 
nature  has  been  considered  by  some  botanists  to  be  a 
genuine  nucleus.  They  resemble  the  bacteria,  also,  in 


THE   SIMPLEST   FORMS   OF  LIFE  37 

being  exceedingly  resistant  to  extremes  of  heat  and 
cold  that  would  be  fatal  to  most  plants  of  higher 
organization.  They  often  occur  in  thermal  springs 
which  are  impregnated  with  various  substances  usually 
inimical  to  plant  life.  The  name  Cyanophycese  has 
been  given  to  this  class,  because  in  addition  to  the 
chlorophyll  they  usually  possess  a  blue  pigment  (phyco- 
cyanin)  which  is  readily  soluble  in  water. 

The  similarity  in  the  structure  and  reproduction  of 
the  Cyanophyceae  and  Bacteria  have  led  botanists  to 
unite  them  into  a  common  group,  the  Schizophyta, 
based  upon  the  prevailing  method  of  reproduction  by 
simple,  transverse  fission.  Whether  this  group  is  di- 
rectly related  to  any  other  group  of  plants  is  question- 
able ;  but  there  is  good  reason  to  suppose  that  they 
represent  an  extremely  primitive  type  of  vegetation, 
and  it  has  even  been  suggested  that  similar  organisms 
were  probably  among  the  first  to  make  their  appearance 
upon  the  earth  before  the  conditions  were  fit  for  higher 
forms  of  plant  life. 

It  seems  probable  that  the  earliest  forms  of  life  could 
manufacture  carbon-bearing  compounds  without  pos- 
sessing chlorophyll,  and  that  the  restriction  of  this 
power  to  green  cells  is  a  secondary  condition.1 

While  both  the  Slime-moulds  and  Schizophytes  show 
but  doubtful  affinity  with  the  higher  plants,  there  is  a 
third  group  of  low  organisms,  sometimes  united  with 
these  two  under  the  name  of  Protophytes,  which  are 
of  especial  interest  in  connection  with  the  evolution  of 

1  Certain  bacteria,  although  destitute  of  chlorophyll,  are  independent 
of  organic  food.  Such  forms,  however,  possess  a  red  or  purple  pig- 
ment, which  serves  as  a  substitute  for  chlorophyll. 


38  EVOLUTION   OF   PLANTS 

the  higher  plants.  These  are  known  as  the  Volvoca- 
cetfi,  or  Volvocinese,  and  have  been  claimed  by  zoolo- 
gists as  animals,  although  there  seems  no  question  of 
their  close  relationship  with  the  lower  green  plants. 
It  is  true  that  they  are  actively  motile,  and  show  other 
animal  properties,  but  they  usually  possess  a  cellulose 
membrane,  and  the  characteristic  green  chromatophore 
of  the  typical  plant  cell,  and  inasmuch  as  they  are  con- 
nected with  unmistakable  plants  by  a  complete  series 
of  intermediate  forms,  there  seems  to  be  110  valid 
reason  for  not  considering  them  as  low  plants.  It 
is  interesting  to  note,  however,  that  the  lower  mem- 
bers of  the  series  of  Volvocinese  are  very  much  like  the 
animal  flagellate  infusorians  and  also  the  swarm-spores 
of  the  slime-moulds,  from  which  they  differ  mainly  in 
the  presence  of  a  green  chromatophore.  The  frequent 
recurrence  of  this  free-swimming,  flagellate  type  among 
both  the  lower  animals  and  plants  suggests  some  similar 
forms  as  the  ancestral  type  for  both  of  the  great  series 
of  organic  beings,  which  here  converge. 

The  simplest  of  the  Volvocineae  are  round  or  oval 
cells,  which  in  their  ordinary  condition  are  actively 
motile,  swimming  by  means  of  two  delicate  cilia.  In 
the  younger  stages  these  cells  are  quite  destitute  of  a 
membrane,  but  older  cells  usually  have  a  distinct  cellu- 
lose wall,  with  openings  through  which  the  two  cilia 
protrude.  The  structure  (Fig.  6,  B)  is  that  typical 
of  the  lower  green  plants.  The  green  chromatophore 
(chloroplast)  has  the  form  of  a  cup  and  fits  into  the 
lower  part  of  the  oval  cell-cavity.  Within  the  hollow 
of  the  chromatophore  is  included  a  mass  of  protoplasm 
in  which  is  imbedded  the  nucleus.  The  forward  end 


THE   SIMPLEST   FORMS   OF   LIFE 


39 


of  the  cell  is  occupied  by  colorless  protoplasm,  and 
near  the  outside  is  a  bright  red  pigment-mass  —  the 
" eye-spot"  (<?),  near  which  can  generally  be  detected 
one  or  two  pulsating  or 
contractile  vacuoles, 
such  as  frequently  oc- 
cur in  the  lower  uni- 
cellular animals  and  in 
the  slime-moulds. 

If  these  free-swim- 
ming green  cells  are 
placed  in  a  glass  vessel 
full  of  water,  and  placed 
where  they  are  more 
strongly  illuminated 
from  one  side,  as  for 
example  in  a  window, 
it  will  be  found  that 
very  soon  they  collect 
on  the  lighted  side,  and, 
if  they  are  present  in 
large  numbers,  may  be 
seen  to  form  a  green 
line  close  to  the  side 
where  the  light  is 
strongest.  There  is  rea- 
son to  suppose  that  the 
red  eye-spot  is  in  some 
way  connected  with 
this  sensitiveness  to 

light,  as  it  is  nearly  always  present  in  those  motile 
green  cells  which  show  sensitiveness  to  light,  and  is 


FIG.  6  (Volvocacese).  —  A,  a  plant  of 
Pleodorina  Californica,  showing 
the  ciliated  cells  of  which  it  is  com- 
posed ;  the  arrow  shows  the  direction 
in  which  it  moves;  B,  one  of  the 
smaller  cells,  much  enlarged,  showing 
the  two  long  cilia,  c,  the  eye-spot,  e, 
the  nucleus,  n,  the  pyrenoid,  p,  im- 
bedded in  the  cup-shaped  chloroplast, 
cl ;  C,  three  stages  in  the  division  of 
one  of  the  large  cells ;  D,  the  egg ;  E, 
spermatozoid  of  Volvox ;  'F,  two  ga- 
metes of  Pandorina  fusing  together 
to  form  the  zygote,  or  resting-spore. 
(Figs.  B,  C,  after  Shaw;  D,  E,  after 
Overtoil;  F  after  Pringsheim.) 


40  EVOLUTION  OF  PLANTS 

always  at  the  end  which  is  directed  toward  the  light 
when  they  are  in  motion.  Moreover,  in  the  multicel- 
lular  forms  (Fig.  6,  A)  it  is  those  cells  which  are  in 
the  forward  part  of  the  colony  which  have  the  eye -spot 
best  developed. 

The  multiplication  of  the  lower  Volvocinese,  i.e.  the 
unicellular  forms,  is  accomplished  by  an  internal  divi- 
sion of  the  cell-contents  after  the  withdrawal  of  the 
cilia  and  the  development  of  a  firm  cell-membrane. 
This  division  is  accompanied  by  a  preliminary  division 
of  the  nucleus  and  chromatophore,  but  the  eye-spot  and 
contractile  vacuoles  are  probably  formed  anew  in  the 
daughter-cells.  The  latter  escape  from  the  mother- 
cell  and,  developing  cilia,  become  at  once  complete 
individuals. 

In  the  higher  members  of  the  group,  like  Pleodorina 
(Fig.  6,  A),  the  plant  is  multicellular,  and  the  new 
individual  arises  by  the  repeated  fission  of  the  *mother- 
cell,  but  the  resulting  cells  remain  connected,  and  form 
a  multicellular  complex  of  definite  form,  each  cell  of 
which  has  the  structure  of  the  simpler  unicellular 
forms.  In  some  of  these  multicellular  genera  the  cells 
are  all  alike,  and  are  at  the  same  time  vegetative  and 
reproductive,  any  cell  having  the  power  of  dividing 
repeatedly  and  thus  giving  rise  to  a  new  plant.  In  the 
most  specialized  forms  in  the  group,  such  as  Pleodorina, 
each  individual  has  cells  of  two  kinds,  small,  purely 
vegetative  ones  and  large,  reproductive  ones.  In  the 
genus  Volvox  only  a  small  number  of  cells  have  the 
power  of  dividing,  and  these  have  completely  lost 
the  cilia  and  eye-spot.  Even  in  the  largest  specimens, 
where  the  vegetative  cells  number  several  thousand, 


THE   SIMPLEST  FORMS   OF  LIFE  41 

there  are  not  more  than  a  dozen  of  these  large 
gonidia  or  reproductive  cells. 

Within  the  group  of  the  Volvocinese  there  is  very 
perfectly  exhibited  the  evolution  of  the  sexual  cells. 
The  lowest  members  of  the  series  show  no  marked  differ- 
ences between  vegetative  and  reproductive  cells,  and 
the  latter  are  much  the  same  whether  they  are  sexual 
or  non-sexual.  Thus,  in  the  genus  Pandorina  (Fig.  6, 
F),  the  sexual  cells  are  hardly  distinguishable  from  the 
vegetative  ones,  or  those  which  give  rise  to  a  new  in- 
dividual by  simple  fission ;  but  these  sexual  cells  sepa- 
rate, and,  escaping  from  the  colony,  swim  about  as 
unicellular  individuals  for  a  short  time.  Two  of  these 
free-swimming  cells  then  come  together  and  fuse  into 
a  single  one  which  becomes  later  a  resting-spore,  which 
in  time  will  give  rise  to  new  individuals.  This  fusion 
of  two  similar  cells  is  the  simplest  type  of  sexual 
reproduction. 

In  the  higher  Volvocineae,  there  is  a  gradual  differ- 
entiation of  the  reproductive  cells,  at  first  indicated  by 
a  slight  difference  in  the  size  of  the  male  and  female 
cells,  which  are  much  alike ;  but  in  the  genus  Volvox, 
which  is  the  highest  of  the  series,  the  male  cell  is  very 
small  and  ciliated,  and  is  now  called  a  spermatozoid  (E), 
while  the  female  cell  is  very  much  larger  and  quite 
destitute  of  motion.  This  large,  non-motile  cell  is  called 
the  egg,  or  germ-cell  (D). 

If  we  compare  the  different  members  of  this  order 
we  find  them  forming  a  continuous  series  in  which  dif- 
ferentiation has  proceeded  in  two  directions,  while  all, 
nevertheless,  retain  the  primitive  power  of  active  loco- 
motion. While  the  lower  members  of  the  series  are 


42  EVOLUTION   OF  PLANTS 

strictly  unicellular,  the  higher  ones  are  multicellular, 
this  being  most  marked  in  the  genus  Volvox,  where  there 
are  several  thousand  cells  in  each  individual,  all  con- 
nected by  protoplasmic  threads.  We  have  seen,  too, 
that,  in  the  simpler  types,  all  the  cells  are  alike,  and 
there  is  no  clear  distinction  between  vegetative  and 
reproductive  cells,  nor  between  sexual  and  non-sexual 
ones,  while  in  the  higher  ones  special  cells  are  set  apart 
for  reproductive  purposes,  and  sexuality  is  well  marked. 
It  must  be  remembered,  however,  that  the  evolution 
of  the  plant  body  in  such  specialized  Volvocinese,  as 
Volvox,  is  in  a  direction  away  from  that  which  leads  up 
to  the  more  typical  plants,  and  there  is  no  evidence 
that  this  peculiar  line  of  development  has  ever  advanced 
beyond  such  forms  as  Volvox,  which  seems  to  represent 
the  highest  expression  of  this  type  of  structure.  We 
must  regard  the  whole  series  of  the  Volvocinese  as  an 
offshoot  of  the  main  line  of  development  of  plants. 
The  simplest  of  the  group,  such  as  Chlamydomonas,  are 
closely  related  to  the  lowest  of  the  typical  green  plants, 
the  so-called  Protococcacese,  and  may  be  considered  to 
represent  a  primitive  stock  which  has  given  rise  to  two 
branches,  one,  the  Volvocineae,  culminating  in  Volvox, 
the  other,  the  Protococcaceee,  which  leads  directly  to 
the  higher  green  plants. 

THE   PEOTOCOCCACE^E 

The  Protococcacese,  employing  this  term  in  its  widest 
sense,  form  a  rather  poorly  defined  group  of  unicellular 
plants,  some  of  which  are  of  doubtful  autonomy,  since 
many  supposed  members  of  this  group  have  been  shown 


THE   SIMPLEST  FORMS  OF  LIFE 


43 


to  be  merely  stages  in  the  development  of  higher  algse, 
which,  nevertheless,  may  grow  independently  for  a  long 
time,  giving  rise  to  many 
generations  of  unicel- 
lular individuals  before 
the  definitive  form  is 
reached.  Many  of  the 
Protococcacese,  however, 
such  as  the  curious 
water-net  (Hydrodicty- 
on)  (Fig.  7,  B,  C),  are 
unquestionably  dis- 
tinct. 

The  lowest  members 
of  the  group,  like  Pleu- 
rococcus  (Fig.  7,  A), 
recall  in  structure  very 
strongly  the  resting- 
stages  of  many  Volvo- 
cinese,  and  it  is  interest- 
ing to  note  that  in  most 
of  the  Protococcacese 
the  reproductive  cells 
are  actively  motile,  and 
closely  resemble  the  ac- 
tive cells  of  the  Volvo- 
cinete.  These  reproduc- 
tive cells  are  generally  formed  by  internal  divisions 
of  the  protoplasm  of  the  mother-cell,  from  which 
they  escape  in  the  form  of  biciliate  naked  cells  almost 
identical  with  the  Volvox  cell.  These  motile  cells 
soon  come  to  rest,  become  invested  with  a  cell-wall, 


FIG.  7  (Protococcaceae) .  —  A,  Pleurococ- 
cus,  one  of  the  unicellular  Protococ- 
cacese;  L,  a  full-grown  individual ;  II., 
III.,  division  stages;  B,  part  of  a 
very  young  water-net  (Hydrodictyon) , 
formed  of  coherent  unicellular  indi- 
viduals, each  with  a  single  nucleus 
and  chloroplast;  C,  part  of  a  much 
older  net,  less  highly  magnified;  each 
cell  has  many  nuclei,  and  the  chloro- 
plast has  broken  up  into  many  parts ; 
D,  E,  Pediastrum ;  D,  a  full-grown 
colony ;  E,  young  colony,  the  individ- 
ual cells  still  separate,  but  the  whole 
enclosed  in  the  membrane  derived  from 
the  wall  of  the  mother-cell. 


44  EVOLUTION  OF  PLANTS 

and  soon  assume  all  the  characters  of  the  parent- 
cell. 

In  the  more  specialized  forms,  like  the  water-net 
(Fig.  7,  B,  C),  which  may  be  said  to  bear  somewhat 
the  same  relation  to  the  lower  forms  that  Volvox  does  to 
the  lower  Volvocinese,  the  young  individuals  become 
united  into  a  colony  of  definite  form.  The  zoospores, 
or  ciliated,  reproductive  cells,  remain  within  the  mother- 
cell,  where  they  grow  together  into  a  small  net  which 
is  set  free  by  the  gradual  dissolution  of  the  wall  of  the 
mother-cell.  The  sexual  cells,  however,  are  ejected 
from  the  mother-cell,  and  unite  two  and  two  into 
spores  which,  in  time,  after  several  intermediate  stages, 
give  rise  to  several  new  nets.  In  nearly  all  these 
forms,  both  non-sexual  and  sexual  cells  show  a  rever- 
sion to  the  primitive  biciliate  cell,  resembling  closely 
in  its  structure  the  individual  cells  of  the  lower 
Volvocinese  from  which  these  forms  have  presumably 
originated,  but  having  only  a  very  limited  period  of 
independent,  active  existence. 

Within  the  Protococcacese  we  find  considerably  less 
specialization  than  exists  among  the  Volvocinese.  Thus 
none  of  them  can  be  properly  considered  as  truly  multi- 
cellular,  for  such  forms  as  the  water-net  and  its  allies  are 
really  colonies  of  originally  unicellular  units,  and  all 
the  individuals  of  the  colony  are  alike.  So,  too,  sexual 
cells,  when  they  exist  at  all,  are  of  the  simplest  type, 
with  no  difference  between  the  male  and  female  cells 
or  gametes,  which  closely  resemble  the  non-sexual 
zoospores. 

As  the  lower  Protococcaceae  are  very  intimately  con- 
nected with  the  series  of  green  algae  which  are  the  un- 


THE   SIMPLEST  FORMS  OF  LIFE  45 

doubted  progenitors  of  the  higher  plants,  their  evident 
relationship  with  the  Volvocinese  points  to  the  latter  as 
the  starting  point  for  the  whole  series  of  green  plants 
above  the  Schizophycese. 

SUMMARY 

We  have  seen  that  at  the  bottom  of  the  two  great 
series  of  organisms  —  plants  and  animals  —  there  is  an 
assemblage  of  extremely  simple  forms,  many  of  which 
have  not  reached  a  stage  of  differentiation  where  it  is 
possible  to  say  that  they  are  either  plant  or  animal. 
Of  these  the  Slime-moulds  and  Monera  represent  two 
evidently  related  branches  of  a  common  stock,  which  is 
perhaps,  on  the  whole,  more  animal  than  vegetable  in 
its  nature.  The  slime-moulds,  however,  owing  to  their 
aerial  habits  have  developed  fructifications  which  super- 
ficially closely  resemble  the  sporangia  of  many  higher 
plants,  and  the  spores  are  also  plant-like  in  character. 
These  spores  on  germinating  give  rise  to  uniciliate, 
monad-like  zoospores,  which  closely  resemble  the  lower 
flagellate  infusorians  and  suggest  a  relationship  with 
them.  The  relationship  of  the  slime-moulds  to  the 
other  plants  must  be  considered  extremely  doubtful. 

The  second  group  of  low  plants,  the  Schizophytes, 
while  certainly  to  be  considered  as  plants,  are  neverthe- 
less very  widely  separated  from  any  forms  above  them, 
and  their  connection  with  these  higher  plants  must 
also  be  considered  as,  at  least,  uncertain.  Whether 
the  forms  with  chlorophyll  among  the  Schizophytes  are 
to  be  considered  as  the  primitive  types  from  which  the 
bacteria  or  those  forms  without  chlorophyll  have  been 


46  EVOLUTION   OF   PLANTS 

secondarily  derived,  cannot  be  settled,  but  it  is  ex- 
tremely likely  that  the  whole  group  is  a  very  ancient 
one  and  adapted  to  conditions  quite  unsuited  to  ordi- 
nary types  of  vegetation.  The  presence  of  cilia  in  many 
of  the  bacteria  suggests  possible  affinity  with  the  primi- 
tive motile  forms  from  which  the  higher  plants  have 
originated. 

Finally  we  have  to  consider  the  third  group  of  these 
low  plants,  the  Volvocinese,  which,  while  showing 
marked  similarity  to  the  lower  animals  in  the  actively 
motile  vegetative  cells,  still  in  the  possession  of  a  cellu- 
lose membrane  and  definite  green  chromatophore,  as 
well  as  in  their  nutrition  and  reproduction,  are  typical 
plants.  Within  this  group  there  is  considerable  differ- 
entiation of  the  plant  body  and  the  reproductive  cells, 
but  it  is  among  the  lowest  members  of  the  group  that 
we  are  to  look  for  the  point  of  contact  with  the  higher 
plants  as  well  as  possibly  with  the  lower  animals. 

By  the  loss  of  active  motion  we  may  assume  that 
forms  like  the  lower  Protococcacese  arose,  the  firm 
cellulose  investment  of  the  cell,  found  in  most  plants, 
precluding  the  active  movements  typical  of  the  lower 
animals  and  of  the  Volvocineae.  This  stationary  green 
cell,  with  its  definite  cell-membrane,  may  be  properly 
considered  as  the  starting-point  for  the  series  of  Green 
Algae,  or  Chlorophycese,  which  in  their  turn  are  the  pro- 
genitors of  the  much  more  perfect  green  land  plants. 

The  two  series,  the  Volvocineye  and  Protococcacese, 
may  be  looked  upon  as  offshoots  of  a  common  an- 
cestral type,  probably  resembling  the  existing  uni- 
cellular Volvocinese.  In  one  direction  development 
has  proceeded  without  loss  of  motion  in  the  cells,  re- 


THE   SIMPLEST  FORMS   OF   LIFE  47 

suiting  in  the  higher  types  of  Volvocinese,  like  Volvox ; 
in  the  other,  by  the  loss  of  motility,  there  have  first 
arisen  non-motile  unicellular  plants  like  Pleurococcus, 
from  which  later  have  been  developed  the  multicellular 
green  algse. 


CHAPTER  IV 

ALG^E 

ABOVE  the  Schizophytes  and  Mycetozoa  is  a  large 
assemblage  of  plants,  sometimes  all  united  under  the 
name  of  Thallophyta,  but  probably  better  divided  into 
two  divisions  of  equal  value,  to  which  the  rank  of  sub- 
kingdom  may  with  propriety  be  applied.  The  primary 
division  is  based  upon  the  presence  or  absence  of 
chlorophyll,  and  although  a  few  of  them  which  have 
no  chlorophyll  are  structurally  similar  to  certain  green 
forms,  and  possibly  to  be  considered  as  derived  from 
them,  most  of  the  forms  without  chlorophyll  drffer  pro- 
foundly in  their  structure  from  all  green  plants,  and 
may  properly  be  relegated  to  a  sub-kingdom  of  their 
own.  To  the  forms  which  '  possess  chlorophyll,  the 
name  of  Algae  has  been  given,  while  those  where 
chlorophyll  is  absent  are  known  as  Fungi.  The  two 
low  groups  of  green  plants,  —  the  Protococcacese  and 
Volvocinese,  —  which  were  considered  in  the  last  chap- 
ter, are  usually  included  with  the  Algse,  and  very  prop- 
erly so,  as  they  doubtless  represent  the  lowest  members 
of  the  sub-kingdom. 

The  Algse  are  readily  divisible  into  three  main  divi- 
sions or  classes,  which  are  easily  distinguished  by  their 
color.  All  of  them  possess  chlorophyll,  but  in  two 
of  the  classes  there  are  other  pigments  present,  giving 

48 


ALG.E  49 

them  respectively  a  more  or  less  pronounced  red  or 
brown  color.  On  this  basis  of  color,  the  three  classes 
are  denominated  the  Green  Algae  (Chlorophycese), 
Brown  Algse  (Phseophycese),  and  Red  Algse  (Rhodo- 
phycese).  While,  at  first  sight,  it  would  seem  that 
such  a  classification  is  an  artificial  one,  it  is  found,  on 
more  careful  study,  that  these  color  differences  are 
associated  with  constant  and  characteristic  differences 
of  structure,  which  really  make  the  division  a  very 
natural  one.  Of  these  three  classes,  the  two  latter  are 
mainly  marine,  and  the  peculiarities  and  color  and 
structure  are,  with  little  question,  largely  the  result  of 
their  peculiar  environment. 

THE  GREEN  ALG^E  (CMorophycecB) 

The  Green  Algse  are  for  the  most  part  fresh-water 
plants,  and  although  most  of  them  are  more  compli- 
cated in  structure  than  the  very  simple  Protococcacese 
and  Volvocinese,  still  as  a  whole  the  members  of  the 
class  are  of  simple  structure,  and,  so  far  as  the  vege- 
tative parts  are  concerned,  much  inferior  to  their 
larger  red  and  brown  relatives.  In  spite  of  the  low 
organization  of  the  green  algse,  it  is  among  these, 
rather  than  among  the  more  complicated  and  larger 
marine  red  or  brown  ones,  that  we  must  look  for  the 
ancestors  of  the  lowest  green  land  plants,  —  the  Mosses, 
—  as  there  is  strong  evidence  that  these  originated  from 
aquatic  plants  allied  to  certain  existing  green  algse. 

In  spite  of  their  simplicity,  the  latter  show  a  consid- 
erable degree  of  variation  among  themselves,  both  as 
to  their  vegetative  and  reproductive  parts,  and  upon 


50  EVOLUTION   OF   PLANTS 

these  differences  there  are  based  several  well-defined 
orders. 

Attention  has  already  been  called  to  the  probable 
origin  of  the  higher  green  algse  from  the  Volvocinece, 
and  we  have  seen  how,  by  the  loss  of  free  locomotion, 
the  latter  gave  rise  to  the  simpler  Protococcacese,  which, 
however,  give  an  indication  of  their  origin  from  motile 
ancestors  by  the  frequent  reversion  to  the  primitive 
free-swimming  condition  in  their  reproductive  cells. 

Tracing  up  the  line  of  ascent  in  the  green  algae  a 
step  further,  there  is  found  a  group  of  forms  which 
consist  of  rows  of  perfectly  uniform  cells,  all  alike  and 
individually  closely  resembling  in  structure  the  unicel- 
lular Protococcacese.  Sometimes,  among  the  simpler 
forms  of  filamentous  algge,  it  is  not  uncommon  to  have 
the  filaments  break  up  into  separate  cells,  giving  rise 
to  colonies  of  unicellular  individuals  which  are  not  to 
be  distinguished  from  true  Protococcaceee.  It -is  easy 
to  see  how  the  latter,  by  the  repeated  division  of  a  cell 
in  a  single  plant,  without  separation  of  the  daughter- 
cells,  could  give  rise  to  a  simple  cell-row  or  filament 
such  as  really  makes  up  the  plant  body  in  many  Chloro- 
phycese.  Indeed,  if  we  follow  the  life-history  of  some 
of  these,  we  find  that  its  individual  development  fol- 
lows very  closely  what  we  may  suppose  has  been  the 
history  of  the  whole  group.  Thus  the  non-sexual  re- 
productive cells  are  very  commonly  free-swimming  cells 
(zoospores),  which  show  exactly  the  structure  of  the 
lower  Volvocinese  (Fig.  8,  C).  Such  zoospores  are 
often  biciliate,  possess  a  single  chromatophore,  eye- 
spot,  and  contractile  vacuoles*  and  are  veiy  sensitive 
to  light,  collecting  quickly  on  the  lighted  side  of  the 


ALG^E  51 

vessel  in  which  they  are  placed.  After  a  short  period 
of  active  movement,  they  settle  down,  become  invested 
with  a  cell-membrane,  and  enter  what  may  be  called 
the  Protococcus  stage,  in  which  they  sometimes  remain 
for  a  long  time,  giving  rise  to  large  colonies  of  unicellu- 
lar plants  by  repeated  fission  and  separation  of  the  cells. 
Indeed  these  unicellular  stages  of  many  algse  have  been 
given  special  names,  under  the  mistaken  impression  that 
they  were  really  autonomous  forms  instead  of  simply 
transitory  stages  in  the  development  of  a  filamentous 
al§a.  Usually,  however,  the  zoospore,  after  coming 
to  rest,  elongates,  and,  by  the  formation  of  a  transverse 
wall,  becomes  two-celled,  and,  by  further  elongation 
and  repeated  cross-divisions,  assumes  the  filamentous 
form  of  the  adult  plant.  (Fig.  9,  D,  E,  F.) 

THE   CONFERVACE^E 

The  order  which  seems  to  be  most  directly  connected 
with  the  Protococcaceae  is  that  known  as  the  Confer- 
vacese,  especially  important  in  a  study  of  the  evolution 
of  plants,  as  it  probably  represents,  more  nearly  than  any 
other  existing  group,  the  direct  ancestral  forms  of  the 
higher  plants. 

The  lowest  members  of  the  order  are  simple  un- 
branched  filaments,  composed  of  perfectly  similar  cells 
(Fig.  8,  A).  Somewhat  higher  in  point  of  develop- 
ment are  a  number  of  common  forms,  e.g.  Chsetophora, 
Cladophora  (Fig.  8,  B),  which  are  branched,  while  many 
of  these,  as  well  as  such  of  the  unbranched  forms  as 
are  attached,  often  show  a  modification  of  the  basal  cell 
into  a  root-like  organ.  Where  this  is  the  case  of  course 


52 


EVOLUTION   OF   PLANTS 


the  filament  shows  a  distinction  between  base  and  apex. 
The  most  specialized  forms,  e.g.  Coleochsete  (Fig.  10), 
have  the  form  of  a  flattened  disk,  and  recall  somewhat 

the  structure   found  in 
the  simplest  mosses. 

Within  the  Confer- 
vacese  there  is  found  a 
similar  advance  in  the 
reproductive  parts  to 
that  described  in  the 
Volvocinese.  Some  of 
the  lowest  forms  have  as 
yet  shown  only  non-sex- 
ual reproductive  cells, 
but  it  is  not  improbable 
that  sexuality  will  be 


FIG.  8  (Confervacese). —  A,  a  filament 
of  Microspora,  composed  of  entirely 
uniform  cells ;  B,  part  of  a  plant  of 
Cladophora,  showing  the  branching 
habit ;  C,  a  zoospore  of  Cladophora, 
showing  the  two  cilia,  the  eye-spot, 
e,  and  the  nucleus,  n ;  D,  gametes, 
or  sexual  cells,  of  Ulothrix,  showing 
the  process  of  conjugation.  (Fig.  D, 
after  Dodel.) 


shown  for  all  of  them. 
In  these  forms*  where 
only  non-sexual  zo- 
ospores have  been  ob- 
served, they  may  be 

either  uniciliate  or  biciliate.  These  zoospores  may  arise 
singly,  by  the  escape  of  the  whole  of  the  contents  of 
a  cell  as  a  single  zoospore ;  or  there  may  be  a  division 
of  the  cell-contents  into  two  or  more  parts  which  then 
escape.  When  the  zoospores  are  large  (Fig.  9,  D),  they 
sometimes  have  more  than  two  cilia,  but  otherwise 
resemble  closely  the  typical  Volvox  cell. 

The  simplest  type  of  sexual  reproduction  among  the 
Confervacese  consists  in  the  formation  of  cells  (gametes), 
which  differ  from  the  zoospores  only  in  being,  as  a  rule, 
smaller,  but  with  no  distinction  of  sex.  These  gametes 


ALG^E 


53 


are  always  biciliate,  and  are  set  free  in  the  water  where 
they  unite  in  pairs  to  form  a  single  cell  (zygote)  with 
four  cilia  (Fig.  8,  D),  which  either  at  once  grows  into 
a  new  plant,  or  first 
passes  into  a  resting 
stage  (spore),  which 
then  gives  rise  to  new 
individuals  by  first  form- 
ing one  or  more  zoo- 
spores. 

In  the  higher  mem- 
bers of  the  order,  the  so- 
called  oogamous  forms, 
there  is  a  sharp  separa- 
tion of  the  sexual  cells, 
the  female  cell  becoming 
here  a  large  passive  cell, 
the  egg-cell,  usually 
borne  in  a  specially 
modified  and  enlarged 
cell  called  the  oogonium 
(Fig.  9,  og).  In  the 
form  figured,  the  egg 
closely  resembles  in  its 
formation  and  structure 
the  large  zoospores,  with 
which  it  agrees  except 
in  the  absence  of  cilia, 
and  there  is  no  question  that  here  also  the  gametes  are 
modifications  of  originally  non-sexual  zoospores.  The 
male  gametes  (spermatozoids)  in  these  oogamous  Con- 
fervacese  are  also  borne  in  special  cells  (antheridia)  (Fig. 


FIG 


(Conf  ervacese) .  —  A,  B,  por- 
tions of  two  female  plants  of  (Edo- 
gonium  ;  og,  the  oogonium  ;  in  A,  the 
egg-cell  has  not  yet  been  fertilized, 
in  B,  the  fertilized  egg  has  become 
transformed  into  a  thick-walled  rest- 
ing-spore  ;  the  spermatozoid  enters 
through  the  pore  at  the  top ;  C,  part 
of  a  male  plant  of  the  same  species, 
showing  the  antheridium,  cm;  D,  a 
zodspore  or  motile  non-sexual  repro- 
ductive cell ;  E,  one-celled  plant  de- 
rived from  a  zoospore  ;  F,  the  lower 
part  of  an  older  plant  showing  the 
root-like  outgrowths  (?•)  of  the  basal 
cell. 


54 


EVOLUTION  OF   PLANTS 


9,  C,  cm),  and  closely  resemble  the  zoospores  except  in 
size,  and  the  partial  or  complete  loss  of  chlorophyll.  The 
spermatozoid  has  a  large  nucleus  with  relatively  little 
cytoplasm,  as  the  nucleus  is  probably  of  the  most  impor- 
tance in  the  act  of  fecundation. 

At  maturity  the  oogonium  opens  and  permits  the  en- 
trance of  the  motile  spermatozoid,  which  at  once  pene- 
trates into  the  egg-cell  where  its  nucleus  fuses  with 

that  of  the  egg, 
thus  fertilizing  it. 
As  the  result  of 
fertilization  the 
egg  becomes  in- 
vested with  a 
heavy  cell-wall  and 
forms  a  resting- 
spore  which  re- 
mains dormant  for 
a  long  period,  and 
is  capable  of  re- 
sisting, unharmed, 


FIG.  10.  —  A,  a  plant  of  Coleochsete  scut  at  a, 
one  of  the  highest  of  the  Conferraceae ;  B, 
fragment  of  another  species,  C.  pulvinata, 
with  an  oogonium,  og;  C,  the  germinating 
spore  seen  in  section,  showing  its  division 
into  a  nearly  globular  cell-mass  ;  each  cell 
later  gives  rise  to  a  single  biciliate  zoospore. 
(Figs.  B  and  C  after  Oltmanns.) 


freezingand  drying 
up. 

In  the  highest 
type  of  all,  repre- 
sented by  the  pecu- 
liar genus  Coleo- 

chaete  (Fig.  10),  the  oogonium,  with  the  contained 
oospore,  becomes,  after  fertilization,  invested  with  a 
protective  covering  formed  by  the  growth  of  adjacent 
cells,  so  that  the  influence  of  the  act  of  fertilization  ex- 
tends beyond  the  egg-cell.  Coleochsete,  as  we  shall  see 


ALG^E  55 

later,  shows  certain  interesting  analogies  with  the  lower 
mosses,  and  is  on  the  whole,  the  type  of  the  green  algse 
which  most  nearly  approaches  them.  When  the  oospore 
germinates,  instead  of  forming  a  plant  like  the  original  at 
once,  or  having  its  contents  divided  into  zoospores  which 
then  germinate,  there  is  first  developed  a  cellular  body 
(Fig.  10,  C),  which  may  be  considered  as  a  small  plant 
of  very  limited  growth,  differing  from  the  normally  de- 
veloped sexual  individuals.  From  each  cell  of  this  small 
plant  is  produced  a  single  biciliate  zoospore,  which  then 
develops  into  a  normal  individual.  This  formation 
from  the  resting-spore  of  an  individual  entirely  devoted 
to  the  formation  of  non-sexual  spores,  from  which  the 
sexual  plants  are  finally  developed,  is  very  much  like 
what  occurs  in  the  life-history  of  the  mosses  which 
probably  arose  from  the  Algse  by  a  further  development 
of  this  alternation  of  sexual  and  non-sexual  plants. 
Where  there  is  a  marked  difference  in  structure  between 
these  phases,  such  as  we  find  in  the  mosses  and  ferns, 
the  terms  gametophyte  and  sporophyte  have  been  ap- 
plied respectively  to  the  sexual  and  non-sexual  phases. 
The  first  indication  of  this  differentiation  is  seen  in  such 
forms  as  QEdogonium  (Fig.  9),  where  the  resting-spore 
on  germination  dbes  not  at  once  produce  a  filament  like 
the  parent  plant,  but  first  divides  into  four  zoospores 
which,  on  escaping,  give  rise  to  as  many  new  individuals. 
While  the  Confervacese  probably  form  the  direct 
line  of  ascent  from  the  Volvocineae  to  the  mosses,  there 
are  several  other  orders  of  green  algse  which,  starting 
from  about  the  same  point,  show  specialization  in  various 
directions.  Two  of  these,  the  Siphonese  and  Conjugated 
are  obviously  related  to  the  other  green  forms,  but  a 


56 


EVOLUTION   OF  PLANTS 


third  order,  the  Characese,  is  made  up  of  very  peculiar 
plants  of  doubtful  affinities. 


THE  SIPHONED 

This  order  contains  a  good  many  types  differing  a 
good  deal  among  themselves  and  showing  in  some  cases 

a  high  degree  of  special- 
ization, 
from    the 
forms     in 
complete 


differ 


They 

other    green 
the     almost 
absence      of 
division     walls     within 
the  plant  body,  although 
they    can    hardly    with 
propriety  be  considered 
as    strictly    unicellular 

FIG.  11.— Part  of  a  plant  of  Caulerpa  Since       the       protoplasm 

plumaris,  one  of  the  Siphonese,  show-  ,    •            -,                       T 

ing  external  differentiation  into  stem,  Contains  a  large  number 

root,  and  leaf  in  a  non-cellular  plant ;  np     rniPlPi        'TUp     WJant 
x,  the  growing  point ;  r,  rootlets. 

may  be  a  simple  tubular 

filament,  or  it  may  be  extensively  branched  and  form  a 
body  of  considerable  size  showing  a  remarkable  degree 
of  external  differentiation,  actually  mimicking  the  struct- 
ure of  the  higher  plants  in  the  development  of  stem, 
leaf,  and  root  (Fig.  11)  ;  but  even  in  such  cases  the 
hollow  cavity  of  the  thallus  is  undivided  by  partition 
walls.  The  wall  is  lined  with  a  protoplasmic  layer  in 
which  are  imbedded  the  numerous  nuclei  and  chloro- 
plasts.  The  division  of  the  nuclei,  of  course,  is  not 
accompanied,  as  in  most  cells,  by  the  formation  of  a 
division  wall. 


57 


The  exact  affinities  of  many  of  the  Siphonese  are  still 
obscure,  and  it  is  by  no  means  impossible  that  the  group 
has  had  a  multiple  origin,  i.e.  all  the  members  of  the 
order  may  not  neces- 
sarily be  genetically 
related,  but  there  may 
have  been  a  develop- 
ment of  this  peculiar 
type  from  several  an- 
cestral forms.  While 
the  lowest  of  the  or- 
der show  much  in 
common  with  the 
Protococcacese,  and 
may,  perhaps,  have 
arisen  from  them, 
others  like  the  com- 
mon genus  Vaucheria 
(Fig.  12)  are  struct- 
urally more  like 
some  of  the  Confer- 
vacese.  There  are  a 
number  of  genera 


among  the  latter 
where  the  elongated 
cells  are  multinucle- 
ate  and  there  is  a 
partial  suppression 


FIG.  12.  —  Vaucheria  sessilis,  one  of  the 
fresh-water  Siphoneae  ;  A,  plant  with 
unopened  antheridium,  an,  and  oogo- 
nium,  og ;  B,  an  older  plant  with  the 
antheridium  empty,  and  the  oogonium 
containing  the  resting-spore,  sp  ;  C,  the 
end  of  a  filament  with  a  zoosporangium ; 
D,  zoospore  showing  the  pairs  of  cilia 
corresponding  to  the  individual  nuclei 
in  its  outer  part;  E,  a  germinating 
zoospore. 


of  the  division  walls, 

nuclear  division  and  cell  division  being  quite  indepen- 
dent of  each  other.  By  the  complete  suppression  of  the 
division  walls  in  forms  like  these,  it  is  conceivable  that 


58  EVOLUTION   OF  PLANTS 

a  thallus  of  the  type  found  in  Vaucheria  may  have 
arisen. 

The  largest  members  of  the  order,  which  reaches  its 
highest  development  in  the  tropical  seas,  generally  have 
the  large  thallus  made  up  of  closely  interwoven,  much- 
branched  filaments,  which,  however,  seldom  show  any 
divisions  except  those  by  which  the  reproductive  organs 
are  cut  off.  In  regard  to  the  latter,  these  large  marine 
Siphoneae  are  less  highly  developed  than  some  of  the 
otherwise  much  simpler  fresh-water  genera. 

Within  the  series  we  find  much  the  same  progression 
in  the  development  of  the  reproductive  parts  that  has 
been  described  in  the  Volvocinese  and  Confervaceae. 
Most  of  them  show  both  non-sexual  and  sexual  reproduc- 
tion, the  latter  being  of  a  low  type  in  the  greater  number 
of  them,  with  little  or  no  difference  between  the  male 
and  female  cells.  The  genus  Vaucheria,  however 
(Fig.  12),  shows  perfectly  differentiated  sexual  cells, 
the  larger  passive  egg-cell  being  retained  within  the 
oogonium,  where  it  is  fertilized  by  the  minute  biciliate 
spermatozoids. 

The  Siphonese  exhibit  great  variety  also  in  the  non- 
sexual  reproduction.  Most  of  them  produce  zoospores, 
which  are  usually  provided  with  two  cilia,  but,  in  the 
case  of  Vaucheria,  are  apparently  multiciliate,  owing 
to  the  fact  that  the  individual  biciliate  zoospores  are 
discharged  in  a  mass  and  never  separate.  Besides 
the  zoospores,  there  are  various  forms  of  non-motile 
spores,  and  the  plants  often  increase  in  number  by  the 
separation  of  a  portion  of  the  thallus.  Indeed  in  Cau- 
lerpa  (Fig.  11)  this  is  the  only  known  method  of  repro- 
duction. 


ALG^E 


59 


THE  CONJUGATE 

The  members  of  the  second  subsidiary  order  of  the 
green  algse,  the  Conjugates,  are  mostly  fresh-water 
plants,  having  very  marked  characteristics,  and  distin- 


FIG.  13  (Conjugates). —  A,  two  filaments  of  Spirogyra  showing  the  be- 
ginning of  conjugation;  each  cell  contains  a  single  large  spiral  chlo- 
roplast  with  the  pyrenoids,  p ;  B  and  C,  later  stages  of  conjugation ;  in 
C  the  contents  of  one  of  the  conjugating  cells  has  passed  completely 
over  into  the  other,  the  united  protoplasmic  masses  forming  the  resting- 
spore,  sp  ;  D,  Closterium,  one  of  the  Desmids  or  unicellular  Conjugate  ; 
p,  a  pyrenoid ;  E,  two  views  of  another  Desmid,  Staurastrum ;  the 
shaded  portion  represents  the  chloroplast;  F,  a  Desmid,  Cosmarium, 
in  process  of  cell-division. 

guished  from  most  of  the  other  algae  by  the  complete 
absence  of  ciliated  cells.  The  most  familiar  members  of 
the  group  are  the  "  pond-scums,"  which  occur  in  large, 
frothy  masses,  floating  in  quiet  water.  The  lowest  of 
the  order  are  the  Desmids,  perhaps  the  most  beautiful 
of  all  unicellular  plants  (Fig.  18). 

The  lowest  of  the  desmids  are  simple  oval  cells,  with 


60  EVOLUTION  OF  PLANTS 

a  single  chromatophore,  and  the  cell  may  be  compared 
structurally  to  that  of  the  Protococcacese  or  Volvocinese, 
and  it  is  probably  from  these  that  the  lower  Conjugates 
have  arisen.  Reproduction  takes  place  in  the  lower  des- 
mids  either  by  the  division  of  the  cells  (Fig.  13,  F),  or 
by  the  fusion  of  two  of  them  into  a  single  cell,  or  spore, 
which  subsequently  by  internal  division  gives  rise  to 
several  new  individuals  very  much  like  the  production 
of  zob'spores  within  the  resting-spores  of  many  Pro- 
tococcacese  and  Confervacese. 

From  these  simple  unicellular  types,  it  is  easy  to  trace 
the  development  of  the  series  in  one  direction,  by 
specialization  of  the  individual  cells,  to  the  higher 
clesmids ;  in  the  other,  by  cohesion  of  the  cells,  to  the 
filamentous  pond-scums.  The  latter,  probably,  do  not 
all  form  one  group,  but  have  originated  from  several 
types  of  unicellular  ancestors,  as  there  are  several  genera 
of  unicellular  desmids,  which,  in  the  form  of  their  pe- 
culiar chloroplasts,  closely  resemble  the  different  genera 
of  the  pond-scums.  Thus  Mesotsenium  closely  resembles 
the  individual  cell  of  the  filamentous  Mesocarpus,  and 
Spirotsenia  bears  the  same  resemblance  to  Spirogyra. 

The  chloroplasts  of  the  Conjugatse  are  always  large 
and  usually  have  the  form  of  a  flattened  band  or  plate 
in  which  are  imbedded  one  or  more  roundish  bodies, 
pyrenoids,  such  as  are  common  in  the  chloroplasts  of 
most  other  green  algse  (see  Fig.  13). 

The  absence  of  motile  reproductive  cells  necessitates 
a  special  contrivance  for  fertilization.  Except  in  a  few 
of  the  lowest  forms  where  the  unicellular  individuals 
fuse  together  completely,  union  of  the  sexual  cells  is 
accomplished  by  the  formation  of  protuberances,  grow- 


ALG^E  61 

ing  out  from  them,  which  unite  to  form  a  tube  con- 
necting the  cells  (Fig.  13,  A,  B,  C).  Sometimes  the 
protoplasm  of  one  of  the  cells  passes  over  into  the  other 
one  and  fuses  with  its  protoplasm ;  or  the  protoplasm 
may  leave  both  cells  and  unite  in  the  middle  of  the 
connecting  tube.  In  either  case  the  result  of  the  fusion 
is  the  formation  of  a  thick- walled  resting-spore  (zygo- 
spore).  This  process  of  conjugation  is  characteristic  of 
the  whole  order,  and,  except  in  the  very  lowest  forms, 
consists  in  a  fusion  of  the  cell-contents  only,  the  wall  of 
the  resting-spore  being  an  entirely  new  one. 

THE  CHAKACE^ 

Probably  no  group  of  green  plants  is  more  puzzling  to 
the  systematist  than  the  Characese,  or  stone-worts,  as 
they  are  sometimes  called,  on  account  of  the  heavy  coat- 
ing of  calcium  carbonate  frequently  deposited  in  their 
outer  cell-walls,  which  renders  the  plant  rigid  and 
brittle.  These  curious  aquatics  are  all  closely  related 
among  themselves,  but  show  no  very  obvious  affinity 
with  any  other  group  of  algse,  and  at  present  all 
attempts  to  connect  them  with  the  other  algse  are  little 
better  than  mere  conjecture. 

All  the  Characese  are  characterized  by  the  regular 
division  of  the  axis  into  nodes  and  internodes  which 
bear  a  definite  relation  to  the  first  divisions  in  the  large 
apical  cell  which  terminates  each  growing  shoot.  The 
plants  are  remarkable  for  the  great  size  of  the  inter- 
nodal  cells,  which  often  reach  a  length  of  several  centi- 
metres with  a  diameter  of  a  millimetre.  The  protoplasm 
of  these  long  cells  shows  a  very  active  rotation  within 


EVOLUTION   OF  PLANTS 


the  cell,  and  the  cells  have  long  been  favorite  subjects 
for  demonstrating  this  phenomenon.  The  original 
nucleus  of  these  elongated  cells  becomes  early  divided 
into  many,  but  these  secondary  nuclei  are  not  formed 


FIG.  14.  —  A,  a  plant  of  Chara,  one  of  the  Characeae,  showing  the  division 
of  the  stem  into  nodes  and  internodes,  and  the  method  of  branching ; 
B,  part  of  a  leaf  with  an  antheridium  an,  and  oogonium,  oy ;  I,  leaflets 
at  the  node  of  the  leaf;  C,  a  group  of  filaments  from  the  interior 
of  the  antheridium ;  each  cell  of  the  long  filament  contains  a  hiciliate 
spermatozoid ;  D,  a  section  through  the  node  of  a  young  leaf  showing 
the  young  antheridium,  an,  below  the  oogonium,  oy ;  E,  a  single 
spermatozoid  ;  F,  a  longitudinal  section  of  the  stem-apex,  showing  the 
apical  cell,  v,  from  the  division  of  which  all  the  parts  of  the  plant 
arise  ;  x,  the  nodes,  y,  the  internodes. 

by  the  ordinary  nuclear  division,  or  karyokinesis,  but 
result  from  a  direct  constriction,  or  fragmentation  of 
the  primary  nucleus,  a  phenomenon  which  has  also 
been  met  with  in  the  elongated  cells  of  the  stems  of 
some  flowering  plants. 


63 

No  special  non -sexual  reproductive  organs  occur  in 
these  plants,  beyond  the  separation  of  small  fragments, 
usually  nodes,  which  may,  under  proper  conditions,  de- 
velop into  new  individuals. 

The  sexual  organs,  antheridia  and  oogonia,  are  ex- 
tremely complicated,  especially  the  former,  and  differ 
very  much  from  those  of  all  other  algse.  They  show 
certain  analogies  with  the  reproductive  organs  of  some 
of  the  lower  mosses,  this  being  especially  the  case  with 
regard  to  the  spermatozoids,  which  are  strikingly  similar 
to  those  of  some  mosses.  A  single  large  spore  results  from 
the  fertilization  of  the  egg-cell,  which  is  surrounded  by  a 
protective  covering  formed  by  a  series  of  cells  about  it. 

The  spore  on  germination  produces  a  simple  conferva- 
like  filament,  or  "  protonema,"  upon  which  the  fully  de- 
veloped plant  arises  as  a  lateral  branch.  As  this  is 
somewhat  like  the  formation  of  the  leafy  stems  in  the 
common  mosses,  it  has  been  suggested  that  there  may 
be  some  genetic  connection  between  the  latter  and  the 
Characese ;  but  this  is  highly  improbable  in  view  of  the 
great  differences  in  the  structure  of  the  plants  of  the  two 
groups,  although  the  analogies  in  the  structure  of  the  re- 
productive organs  may  indicate  a  remote  relationship  be- 
tween them. 

THE  BROWN  AND  RED  ALG.E 

While  the  green  algse  are  for  the  most  part  inhabi- 
tants of  fresh  water,  the  two  other  great  groups  of 
Algse  are  mostly  found  only  in  the  sea,  where  they  con- 
stitute the  most  conspicuous  features  of  the  marine 
flora.  Both  classes  include  plants  of  much  greater  size 
and  complexity  than  any  green  algse,  some  of  the  great 


64  EVOLUTION  OF  PLANTS 

kelps  being  plants  of  gigantic  size.  Both  groups  differ 
in  many  respects  from  the  green  algse,  and  it  is  an  open 
question  whether  they  have  been  derived  from  the  latter, 
or  whether  they  are  to  be  traced  back  to  unicellular 
ancestors,  in  which  their  peculiar  pigments  were  already 
developed.  These  red  and  brown  pigments  are  doubt- 
less associated  with  the  process  of  photo-synthesis,  and 
are  probably  the  results  of  the  peculiar  environment  of 
these  sea-weeds. 


THE  BROWN  ALG^E  (Phceophycece) 

Before  examining  the  more  highly  organized  plants 
to  which  the  term  Phseophycese  is  usually  applied,  it 
may  be  well  to  consider  a  number  of  simple  forms  pos- 
sibly allied  to  them,  and,  although  minute  in  size,  of 
great  importance  in  the  economy  of  nature.  Some  of 
these  are  inhabitants  of  fresh  water,  but  the  greater 
number  are  free-swimming  or  pelagic  organisms  occur- 
ring in  the  open  ocean,  and  forming  an  important  con- 
stituent of  the  so-called  "  plankton  "  or  floating  life  of 
the  ocean. 

The  simplest  of  these  (Fig.  15,  A,  B)  are  very  minute 
ciliated  organisms  recalling  the  green,  fresh-water  Vol- 
vocinese,  and  possibly  related  to  them.  Like  these  they 
show  evident  resemblances  to  the  flagellate  infusorians, 
from  which  they  differ  mainly  in  the  presence  of  chromat- 
ophores,  and  the  absence  of  an  opening  by  which  solid 
food  can  be  ingested.  These  plants  have  chromato- 
phores  which  contain  a  pigment  much  like  that  of  the  true 
Phseophycese,  and  possibly  may  bear  the  same  relation  to 
this  class  that  the  Volvocinese  do  to  the  green  algae. 


ALG^E 


65 


A  second  group  of  unicellular  plants,  resembling  the 
Phseophycese  in  color,  but  otherwise  more  like  some  of 
the  green  algae,  are  the 
Diatoms  (Fig.  15,  C,D),  a 
group  including  many 
thousand  species,  which 
often  occur  in  enormous 
masses.  Although  these 
are  unicellular,  they  are 
often  united  into  colonies 
of  definite  form,  but  more 
commonly  are  free.  The 
chromatophores  are  usu- 
ally two  in  number  and 

flattened  in  shape,  but  may    FIG.  15. -A,  B,   Peridineae;  C,  D, 
*•  "          Diatomaceae.   A,  Hemidimuni  na- 

be  numerous  and  of  the 
round  or  oval  form  com- 
monly found  in  the  higher 
PhseophycesB.  As  in  the 
latter  there  is  present  a 
brown  pigment  (diatomin)  which  quite  conceals  the 
chlorophyll.  A  further  peculiarity  of  these  plants  is 
the  presence  of  a  silicious  shell,  composed  of  two  valves, 
one  fitting  into  the  other  (Fig.  15,  C.,  II).  This  glassy 
case  is  often  sculptured  in  a  most  beautiful  manner,  and 
the  fine  markings  are  favorite  tests  for  microscopic 
lenses.  The  diatoms  often  exhibit  creeping  move- 
ments, but  are  never  ciliated.  The  multiplication  of 
the  diatoms  is  either  by  fission,  or  by  the  formation  of 
so-called  "  auxospores,"  which  may  be  formed  either 
sexually  or  asexually. 

While  diatoms  are  common  in  fresh  water,  it  is  in 


sutum  (after  Stein)  ;  B,  Peridi- 
nium  divergens  (after  Schtitt) ;  C, 
Plnnularia  viridis:  i,  from  above, 
n,  from  the  side,  showing  the  over- 
lapping valves  of  which  the  shell  is 
composed ;  D,  Navicula  sp.  ?  show- 
ing the  two  chromatophores,  d. 


66  EVOLUTION  OF   PLANTS 

the  ocean  that  they  are  of  most  importance.  Here,  es- 
pecially in  the  colder  parts  of  the  sea,  they  form  the 
greater  part  of  the  floating  vegetation,  and  sometimes 
occur  in  such  enormous  masses  as  to  discolor  the  water 
over  wide  areas.  It  is  these  masses  of  floating  unicel- 
lular plants  which  are  the  primary  source  of  food  for 
all  the  hosts  of  animal  life  in  the  ocean,  and  it  is  to 
these  minute  organisms  that  the  manufacture  of  organic 
substances  is  due,  and  they  serve  as  food  for  innumera- 
ble smaller  animals,  and  sometimes  larger  ones  as  well, 
which,  in  their  turn,  are  devoured  by  higher  forms. 

In  the  warmer  waters,  the  diatoms  are  largely  re- 
placed by  the  other  unicellular  plants  already  referred 
to,  as  well  as  others  whose  affinities  are  still  obscure. 
As  the  larger  sea-weeds,  with  few  exceptions,  are  at- 
tached, they  are  of  necessity  confined  to  a  narrow  zone 
of  shallow  water  skirting  the  shore,  and  in  spite  of  their 
large  size,  are  of  slight  importance  as  compared  with 
the  hosts  of  minute  pelagic  plants. 

The  silicious  shells  of  diatoms  are  almost  indestruc- 
tible, and  have  been  preserved  in  a  fossil  condition  so 
that  even  the  species  are  readily  determined.  These 
deposits  are  often  of  great  thickness,  showing  that,  for- 
merly, as  at  present,  these  plants  occurred  in  immense 
numbers.  However,  geologically  speaking,  the  group 
is  not  an  extremely  old  one,  but  appears  somewhat 
suddenly  in  the  later  secondary,  and  early  tertiary  rocks. 

THE  PHJEOPHYCE^E 

The  true  Phseophycese  are  almost  exclusively  marine 
and  form  a  clearly  defined  class  with  no  certain  affinity 


6T 


with  the  other  Algse.  They  include  by  far  the  largest 
of  the  sea-weeds,  and  are  familiar  objects  of  the  sea- 
shore. With  the  exception  of 
a  few  forms  like  the  gulf-weed 
(Sargassum),  which  seems  to  be 
really  a  floating  plant,  they  are 
usually  firmly  attached  to  rocks 
and  other  objects  by  means  of 
highly  developed  root-like  or- 
gans or  holdfasts.  They  may 
grow  where  they  are  completely 
submerged,  but  many  of  them 
occur  between  tide-marks  so 
that  they  are  partially  or  com- 
pletely exposed  at  low  tide.  A 
common  feature  of  many  of 
them  is  the  development  of 
floats,  or  air-bladders  by  which 
the  plant  is  buoyed  up  and 
brought  near  the  surface  and 
thus  exposed  to  the  light. 

Within  this  class  there  is 
great  range  of  structure  as  well 
as  size.  The  simplest  forms  are 
delicate,  branching  filaments 
much  like  many  Confervaceee, 
except  for  their  brown  color. 

Others  are  gigantic  plants  reaching  a  length  of  a  hun- 
dred metres  or  more,  rivalling  the  largest  of  terrestrial 
plants.  As  might  be  expected,  these  giant  kelps  show 
a  considerable  degree  of  specialization  in  their  tissues, 
but  there  is  to  be  found  almost  every  intermediate  con- 


FIG.  1(>.  —  Ectocarpits  yranu- 
losus,  one  of  the  simpler 
Brown  Algae  or  Phaeophyceae  ; 
A,  part  of  a  plant  showing 
the  sporangia,  $p ;  B,  a  uni- 
locular  sporangium,  sp,  more 
highly  magnified  ;  C,  young ; 
D,  older  plurilocular  spo- 
rangium ;  cl,  the  irregular 
chromatophores  ;  E,  a  single 
gamete  of  E.  Siliculosus, 
showing  the  lateral  position 
of  the  cilia.  (Fig.  E  after 
Berthold.) 


68 


EVOLUTION   OF  PLANTS 


dition  between  these  and  the  simplest  types,  like  Ecto- 
carpus  (Fig.  16).     Among  the   most  characteristic  of 

these  larger  forms 
may  be  mentioned  the 
great  bladder-kelps  of 
the  Pacific  (Macro- 
cystis,  Nereoc}rstis) 
(Fig.  17),  and  the 
smaller  Lamin arias  of 
the  Atlantic.  Many 
of  the  larger  kelps 
grow  where  they  are 
exposed  to  the  full 
force  of  the  heavy 
surf,  and  this  ac- 
counts for  the  tough, 
leathery  consistency 
of  many  of  them,  and 
the  powerful  hold- 
fasts or  roots. 

An  examination  of 
the  whole  class  shows 
that  within  it  there 
has  been  much  such 
an  evolution  of  the 
reproductive  cells  as 
we  have  seen  in  sev- 
eral groups  of  the 
green  algse ;  but  this 
is  by  no  means  paral- 
leled by  the  vegetative  parts,  as  the  largest,  and,  so  far  as 
the  plant-body  is  concerned,  the  most  specialized  forms, 


FIG.  17.  —  A  young  plant  of  Nereocystis 
Lutkeana,  one  of  the  large  kelps, 
much  reduced,  showing  the  holdfast, 
r,  and  the  float,  v,  with  the  large 
leaves  at  its  summit ;  the  fully  grown 
plant  may  reach  a  length  of  a  hun- 
dred feet  or  more;  B,  the  simple  uni- 
locular  sporangia,  sp,  and  sterile 
hairs,  or  paraphyses,  pa?',  much  mag- 
nified. 


69 


including  all  the  giant  kelps,  show  only  the  simplest 
possible  form  of  reproduction,  i.e.  purely  non-sexual 
zoospores.  Many  of  these  larger  kelps  show  an  external 
differentiation  which  simulates  closely  the  parts  of  the 
higher  terrestrial  plants.  A  definite  axis,  with  lateral 
leaf-like  outgrowths,  has 
its  base  modified  into  a 
mass  of  firm  root-like  or- 
gans, forming  a  most 
efficient  holdfast  or  an- 
chor, which,  in  some  of 
the  largest  kelps,  when 
torn  away  may  carry 
with  it  a  mass  of  rocks 
and  shells  weighing  sev- 
eral hundred  pounds. 

The  leaves  of  the  large 
kelps  are  often  several 
metres  in  length,  and 
although  structurally 

spermatozoid  more  highly  magnifie 


biciliate    spermatozoids ;  'C,   a  single 


FIG.  18  (Fucacese).  —  A,  a  fragment  of  the 
common  gulf-weed,  Sargassum,  show- 
ing the  definite  stem  and  leaves,  and 
the  berry-like  floats,  v ;  B,  the  egg  of 
the  common  rock-weed  (Fucus  vesi- 
culosus),  being  fertilized  byjthe  minute 

they  differ  widely  from 
those  of  the  higher 
plants,  yet  functionally  they  must  be  considered  as 
equivalent  to  these.  It  is  in  these  organs  that  the 
greater  part  of  the  chlorophyll-bearing  cells  are  situ- 
ated. The  peculiar  floats  or  air-bladders  found  in  these 
plants  are  formed  by  the  accumulation  of  gases  within 
certain  parts  of  the  plant,  resulting  in  a  distention  of 
the  thallus  at  these  points,  but  the  details  of  their 
development  cannot  be  given  here. 

While  some  of  the  forms,  including  the  larger  kelps, 
appear  to  possess  only  non-sexual  zoospores,  others,  like 


70  EVOLUTION   OF    PLANTS 

the  rock-weeds  (Fucus)  (Fig.  18,  B),  and  the  gulf-weed 
(Sargassum)  (Fig.  18,  A),  have  clearly  marked,  sexual 
cells,  large,  non-motile  eggs,  and  small,  ciliated  sperma- 
tozoids,  closely  resembling  the  biciliate  zoospores  of  the 
kelps. 

The  lowest  forms  where  sexual  cells  occur,  i.e.  Ecto- 
carpus,  have  similar  motile  gametes,  while  in  others, 
like  Cutleria,  there  is  a  marked  difference  in  size, 
although  both  gametes  are  motile.  The  most  highly 
specialized  forms,  i.e.  Fucus  and  Sargassum,  produce 
large  non-motile  eggs  and  minute  spermatozoids,  both 
of  which  are  discharged  into  the  water  when  the  egg  is 
fertilized,  in  a  manner  which  recalls  that  of  many  low 
animals,  such  as  the  starfish  or  sea-urchin. 

THE  RED  ALGJE  (Illiodophycece) 

Among  the  most  beautiful  of  all  plants  are  the  Red 
Algse  or  Rhodophycere,  whose  brilliant  colors  and  grace- 
ful forms  are  familiar  to  the  most  superficial  student  of 
the  marine  flora.  They  differ  in  structure  so  much  from 
the  other  Algie,  that  they  are  sometimes  considered  to 
form  a  group  entirely  apart  from  these.  However,  the 
lower  members  of  the  class  show  sufficient  resemblance 
to  the  green  algie  to  make  it  seem  likely  that  there 
is  a  relationship  between  the  two  classes,  although  it  is 
probably  a  remote  one. 

While  not  so  strictly  marine  as  the  typical  Phreophy- 
cese,  still  the  great  majority  of  the  Rhodophycece  occur 
only  in  salt  water.  The  few  members  of  the  class  which 
grow  in  fresh  or  brackish  water  are  insignificant  in  size 
and  dull  in  color,  and  belong  to  the  lower  orders  of  the 
class. 


ALG^E  71 

A  remarkable  characteristic  of  the  class  is  the  absence 
of  the  motile  reproductive  cells  so  common  in  the  brown 
and  green  algse.  In  the  lowest  members  of  the  class, 
the  Bangiacese,  the  reproductive  cells  are  said  to  show 
a  slight  amoeboid  movement,  but  in  all  the  others  even 
such  movement  is  quite  wanting.  Another  peculiarity 
is  the  very  evident  protoplasmic  connections  between 
the  cells  of  the  thallus,  these  being  constantly  present 
in  all  but  the  lowest  types.  These  connections  have 
the  form  of  extremely  delicate  filaments  joining  the 
protoplasmic  bodies  of  adjacent  cells  (Fig.  19,  A). 

They  all  possess  in  addition  to  the  chlorophyll  an 
additional  pigment,  which,  in  most  forms,  is  a  more  or 
less  pronounced  red.  This  pigment  (phycoerythrin) 
is  least  developed  in  the  fresh-water  species,  which 
show  a  more  or  less  decided  green  tinge,  olive  or  black- 
ish rather  than  red.  Many  of  the  salt-water  species, 
however,  show  a  brilliant  rose-red  or  purple  color,  to 
which  they  owe  much  of  their  beauty.  This  red  pig- 
ment is  soluble  in  fresh  water  and  when  it  is  extracted 
from  the  plants  the  chlorophyll-green  becomes  visible. 
The  phycoerythrin  seems  to  be  related  in  its  nature  to 
chlorophyll,  and  probably  is  associated  with  it  in  the 
process  of  photo-synthesis. 

The  red  algae  are  small  plants  compared  to  the 
gigantic  kelps,  but  are  as  a  rule  larger  than  the  green 
algae.  Some  are  exceedingly  delicate,  consisting  of 
simple  or  branching  filaments  much  like  some  of  the 
Confervaceao.  Others  are  composed  of  single  plates  of 
cells,  which  form  an  excessively  delicate,  filmy  thallus. 
Some,  however,  like  the  common  Irish  moss  (Chondrus), 
and  other  species  which  grow  where  they  are  exposed 


72 


EVOLUTION   OF   PLANTS 


to  the  action  of  the  waves,  are  comparatively  large,  and 
tough  and  leathery  in  consistence  like  the  kelps,  which 
they  also  resemble  in  the  general  arrangement  of  the 
cells  in  the  thallus.  One  peculiar  family,  the  so-called 
Coralline  algae,  are  characterized  by  the  deposition  of 
large  amounts  of  carbonate  of  lime,  which  makes  them 


FIG.  19  (Red  Algje).  —  A,  Callithamnion  floccosum,  a  simple  red  sea-weed, 
showing  the  protoplasmic  connections  between  the  cells,  and  the  non- 
sexual  reproductive  bodies,  tetraspores,  sp ;  B,  a  single  tetraspo- 
rangium  with  the  four  contained  spores;  C,  the  spore-fruit,  or  cystocarp 
of  a  somewhat  more  complicated  form,  Polysiphonia ;  D,  one  of  the 
larger  red  sea-weeds,  Gigartina  spinosa,  reduced  about  one-half. 

resemble  corals  in  form,  and  in  their  stony  hardness. 
Some  of  these  often  grow  associated  with  true  corals, 
and  play  an  important  part  in  the  building  up  of  coral 
reefs.  Like  the  true  corals,  these  corallines  have  been 
preserved  very  perfectly  in  a  fossil  condition,  and  they 
appear  to  be  quite  ancient  forms. 


73 

As  a  rule,  the  fresh-water  Rhodophyceaa  are  simpler 
in  structure  than  their  marine  relatives,  and  proba- 
bly represent  a  more  primitive  type  of  structure  from 
which  the  others  have  been  derived.  It  is  not  impos- 
sible that  these  simple  fresh-water  forms  may  also  be 
intermediate  between  the  green  algae  and  the  higher 
Rhodophycese.  It  must  be  admitted,  however,  that,  with 
the  exception  of  the  Bangiacese,  a  group  whose  affinity 
with  the  true  Rhodophycese  has  been  questioned,  all  the 
fresh-water  forms,  although  simpler  in  structure,  are 
typical  Rhodophycese,  so  far  as  the  reproductive  parts 
are  concerned. 

The  motile  zoospores  of  the  brown  and  green  algae 
are  replaced  in  most  Rhodophyceae  by  the  so-called 
tetraspores,  formed  four  together  in  a  common  mother- 
cell,  much  as  zoospores  are  formed.  These  escape  from 
the  mother-cell  and  form  new  plants  at  once  (Fig.  19, 
A,  B). 

The  sexual  reproduction  shows  certain  peculiarities 
which  are  not  found  elsewhere  in  the  vegetable  king- 
dom, although  there  are  certain  analogies  in  the  fertil- 
ization of  some  fungi.  The  antheridium  (Fig.  20,  C) 
is  made  up  of  a  great  number  of  small  cells  which  arise, 
as  short  branches,  very  much  crowded  together.  The 
contents  of  the  terminal  cells  escape  in  the  form  of  a 
naked,  but  non-motile  cell,  or  spermatium,  which  differs 
in  structure  from  the  spermatozoids  of  other  algae, 
mainly  in  the  absence  of  cilia.  So  far  as  is  known,  the 
conveyance  of  the  sperm-cell  to  the  female  reproductive 
organ,  or  procarp,  is  dependent  upon  the  movements  of 
the  water. 

The  female  reproductive  organ  of  the  Rhodophyceae 


•UNIVERSITY 


74 


EVOLUTION   OF   PLANTS 


the  procarp,  or  carpogonium  is,  in  the  lowest  forms  (Fig. 
20,  A),  a  single  cell  much  like  the  oogonium  of  the 
green  algse,  but  there  is  no  contraction  of  the  egg-cell 

preliminary  to  fertil- 
ization. There  is  a 
more  or  less  evident 
prolongation,  known 
*-  as  the  trichogyne  (£), 
developed  from  the 
carpogonium,  and  the 
motionless  sperma- 
tium, on  coming  in 
contact  with  this, 
fuses  with  it  and  the 
walls  of  both  cells  are 
dissolved  at  the  point 
of  contact,  and  the 
contents  of  the  male 
cell  pass  into  the 
trichogyne  and  effect 
fertilization.  It  is 
probable  that  in  most 
cases  there  is  a  fusion 
of  the  nuclei  of  the 
spermatium  and  car- 
pogonium, but  it  has 
been  claimed  that 


FIG.  20.  —Fructification  of  the  Red  Algae ; 
A,  procarp,  or  female  organ  of  one  of 
the  simpler  Rhodophycese,  Batracho- 
spermum ;  t,  the  trichogyne ;  c,  the 
carpogonial  cell ;  B,  the  same  after 
fertilization :  an,  the  spermatium 
united  with  the  trichogyne  ;  sp,  spores 
budding  out  from  the  carpogonial  cell ; 

C,  the  antheridium  of   Polysiphonia ; 

D,  the  multicellular  procarp  of  Sper- 
mothamnium;    t,   the    trichogyne;    E, 
diagram  of  the  procarp  in  the  higher 
Khodpphyceae ;  t,  the  trichogyne  ;  x,  the 
auxiliary    cell    which    is    secondarily 
fertilized    and    produces    the    spores. 
(Figs.  A,   B    after    Davis;    E,    after 
Phillips.) 


sometimes    this    does 
>t  occur,  the  fusion  of  the  protoplasm  being  sufficient 
u)  insure  fertilization.     The  result  of  fertilization  is  not 
a  resting-spore  as  in  the  green  algae,  but  the  carpogo- 
nial cell  sends  out  a  large  number  of  short  branches 


ALG^E  75 

whose  end-cells  are  the  "  carpospores  "  (Fig.  20  B,  sp)  ; 
the  whole  mass  of  spores  budded  off  from  the  fertilized 
carpogonial  cell  forms  the  "sporocarp"  or  "spore-fruit." 

In  the  higher  Rhodophycese,  however,  the  cell  which 
hears  the  trichogyne  does  not  itself  produce  the  spores, 
but  there  are  certain  accessory  cells  (Fig.  20  E,  x)  which 
are  impregnated,  secondarily,  by  outgrowths  from  the 
carpogonial  cell,  known  as  "  ooblastema  filaments."  A 
direct  protoplasmic  connection  is  thus  established  be- 
tween the  carpogonial  cell  and  these  auxiliary  cells, 
whereupon  the  latter  begin  to  bud  freely  and  produce  the 
spores  much  as  these  are  formed  from  the  carpogonial 
cell  in  the  lower  forms.  In  certain  types  the  auxiliary 
cells  are  numerous  and  widely  separated  from  the  car- 
pogonial cell.  In  such  cases  several  very  long  ooblastema 
filaments  grow  out  from  the  latter  after  fertilization,  and 
these  apply  themselves  to  the  auxiliary  cell,  which 
thereupon  produces  a  group  of  spores  in  the  usual 
way.  In  extreme  cases  a  single  ooblastema  filament 
may  be  sufficient  for  impregnating  more  than  one  aux- 
iliary cell. 

From  some  recent  investigations  it  appears  that  some- 
times parthenogenesis  may  occur,  i.e.  the  procarp  may 
give  rise  to  spores  without  fertilization.  How  exten- 
sive this  phenomenon  is,  must  be  determined  by  future 
investigations;  but  the  rarity  of  antheridia  in  some 
species,  and  the  absence  of  spontaneous  movement  in 
the  spermatia  make  it  not  unlikely  that  parthenogene- 
sis is  not  so  rare  a  phenomenon  as  has  usually  been 
supposed.  Among  the  green  algae  parthenogenesis  is 
known  to  occur  in  Ohara  crinita. 


76  EVOLUTION  OF   PLANTS 


SUMMARY 

The  green  algae  are  probably  the  most  primitive 
of  the  three  classes  of  Algae,  and  may  have  given  rise 
to  the  other  two,  although  an  independent  origin  of 
the  red  and  brown  forms  from  unicellular  ancestors  is 
not  impossible,  and  in  the  case  of  the  Phaeophyceae  is 
quite  probable,  as  certain  unicellular  forms,  the  Peri- 
dineae  and  Dinoflagellata  show  a  close  resemblance 
to  the  zoospores  of  the  higher  brown  algae,  and  may 
represent  their  ancestral  forms. 

Among  the  green  algae  the  simpler  Volvocineae  prob- 
ably represent  the  most  primitive  forms  from  which  the 
others  have  sprung.  These  actively  motile  plants  also 
show  possible  affinities  with  such  low  animals  as  the 
flagellate  Infusoria. 

With  this  free-swimming  cell  as  the  starting-point, 
specialization  has  apparently  proceeded  in  several  direc- 
tions. First  of  all,  within  the  group  of  the  Volvocineae 
themselves  there  has  been  specialization  in  two  ways, 
first,  the  production  of  a  multicellular  plant  body ;  sec- 
ond, a  high  degree  of  differentiation  of  the  reproductive 
parts  which  reaches  its  most  complete  expression  in  the 
genus  Volvox.  The  series  of  forms  leading  up  to  the 
latter  is  very  complete,  every  grade  of  development 
being  represented  by  existing  genera. 

The  second  line  of  development  is  illustrated  by  the 
Protococcaceae.  By  the  loss  of  motility  in  the  vegeta- 
tive cells,  and  the  formation  of  a  continuous  cellulose 
membrane,  these  have  lost  their  power  of  locomotion. 
Within  this  series  are  also  found  multicellular  plants, 


ALG^E  77 

but  these  are  more  properly  aggregates  or  colonies  of 
unicellular  units  than  individual  plants.  The  repro- 
ductive cells  of  the  Protococcaceee  are  always  very 
primitive  in  character,  and  usually  are  motile,  much 
like  their  unicellular  volvocineous  ancestors.  The 
third  line  of  development,  represented  by  the  Confer- 
vacese,  may  be  assumed  to  have  arisen  from  the  lower 
Volvocinese  through  the  simpler  Protococcaceee,  with 
which  they  agree  closely  in  their  development.  The 
origin  of  the  typically  inulticellular  Confervacese  from 
the  unicellular  Protococcacese  has  been  brought  about 
by  the  cohesion  of  the  cells  after  fission  is  complete. 
By  repeated  divisions  in  a  single  plane  the  simpler 
filament  of  the  lower  Confervacese  may  thus  be  assumed 
to  have  arisen. 

The  differentiation  of  the  plant  body  first  resulted  in 
the  establishment  of  a  basal  and  apical  region  in  the 
unbranched  filament,  and,  later,  there  arose  branched 
forms  or  a  flat  thallus.  The  development  of  the  repro- 
ductive parts  parallels  very  closely  that  of  the  Volvo- 
cinese, but  the  sexual  cells  in  the  higher  types  are  borne 
in  special  organs,  aiitheridia  and  oogonia. 

The  genus  Coleochsete  is  the  most  specialized  mem- 
ber of  the  order,  and  in  the  formation  of  a  rudimentary 
spore-fruit  (sporophyte)  suggests  a  possible  relationship 
with  the  lowest  mosses. 

The  three  orders — Volvocinese,  Protococcacese,  and 
Confervaceae  —  form,  then,  a  continuous  series  leading 
up  to  the  higher  plants,  while  the  other  algae  are  to  be 
considered  as  offshoots  of  the  main  stock.  Of  these,  the 
Siphonese  have,  perhaps,  had  a  multiple  origin,  the  sim- 
plest one  being  related  to  the  lowest  Volvocinese  or 


78  EVOLUTION  OF  PLANTS 

Protococcacese,  while  others  may  have  sprung  from 
forms  allied  to  the  Confervacese. 

The  second  subsidiary  order  of  green  algse,  the 
Conjugatse,  originated  probably  from  unicellular  forms 
near  the  bottom  of  the  scale,  and  have  retained  a  very 
primitive  type  of  structure,  as  regards  both  the  vege- 
tative and  reproductive  parts. 

In  the  Characese  we  encounter  a  very  circumscribed 
and  specialized  group  of  plants. of  doubtful  affinities, 
showing  no  certain  relationships  with  any  other  groups 
of  algse,  and  possibly  best  removed  from  the  Algse  alto- 
gether and  made  the  type  of  a  special  sub-kingdom. 

Among  the  brown  algse  specialization  has  been  largely 
in  the  direction  of  great  increase  in  size,  accompanied  by 
a  considerable  degree  of  differentiation,  both  of  external 
organs  and  of  the  tissues.  The  evolution  of  the  repro- 
ductive cells  has  not,  in  all  cases,  followed  the  develop- 
ment of  the  plant  body,  and  the  larger  forms,  especially 
the  giant  kelps,  are  in  this  respect  exceedingly  primitive, 
producing  non-sexual  reproductive  cells  only.  Within 
the  class,  however,  there  is  a  development  of  the  sexual 
cells  comparable  to  that  in  the  principal  groups  of  the 
Chlorophycese,  but  even  in  the  highest  types  both  egg- 
cells  and  spermatozoids  are  discharged  into  the  water 
like  the  zoospores  of  the  lower  forms. 

The  red  algae  show  a  marked  divergence  from  the 
Chlorophycese,  not  only  in  their  color,  but  especially  in 
the  complete  absence  of  motile  cells.  In  most  of  them 
the  spores  are  not  formed  directly  from  the  fertilized 
carpogonial  cell,  but  from  certain  auxiliary  cells  which 
are  fertilized  secondarily.  This  is  rendered  possible  by 
the  establishment  of  direct  protoplasmic  connections  be- 


ALG^E 


79 


tween  the  cells.  Parthenogenesis  is  probably  a  not  in- 
frequent phenomenon,  and  marks  the  extreme  point  of 
divergence  from  the  typical  green  algse. 

Both  Phseophycese  and  Rhodophycese  reach  a  far 
higher  degree  of  specialization  of  the  plant  body  than 
is  ever  met  with  among  the  Chlorophycese,  but  there 
is  no  evidence  that  either  group  has  given  rise  to  any 
higher  types  of  plant  structure  than  exist  at  present 
within  the  classes  themselves,  the  most  specialized  of 
the  existing  forms  representing  probably  the  highest 
expression  of  these  peculiar  structural  types. 

The  assumed  relationships  of  the  main  group  of  Algse 
may  be  illustrated  by  the  following  diagram  :  — 


Mo  ses 


Confe  -vaceaa 


Dinoflagellata 
Brown  Alga» 


,occace£8 


Volvocinese 
Green  Alg© 


Siphonea3 


CHAPTER  V 

THE  FUNGI 

ALL  of  the  plants  considered  hitherto,  except  the 
bacteria,  have  been  characterized  by  the  presence  of 
chlorophyll  and  the  accompanying  power  of  assimilat- 
ing carbon  dioxide  as  food.  While  the  latter  property 
may  be  said  to  be  characteristic  of  all  typical  plants,  it 
must  be  remembered  that  there  are  very  many  plants, 
especially  among  the  Thallophytes,  where  this  power 
is  wanting,  and  which  are  quite  destitute  of  any  trace 
of  chlorophyll.  It  is  usually  supposed,  although  this 
is  not  universally  admitted,  that  these  plants  are  the 
descendants  of  green  ancestors,  and  have  lost  their 
chlorophyll  through  the  adoption  of  parasitic  or  sapro- 
phytic  habits,  i.e.  feeding  upon  living  or  dead  organic 
bodies  from  which  they  derive  the  carbon  compounds 
necessary  for  their  growth.  All  of  these  chlorophylless 
plants  below  the  mosses  are  known  as  Fungi,  and  con- 
stitute a  sub-kingdom  which  may  be  considered  to  have 
been  developed  parallel  with  the  Algse,  or  possibly 
may  have  been  derived  from  them.  The  Fungi  are  very 
numerous,  far  exceeding  the  Algae  in  number  of  species. 
Most  of  them  are  probably  plants  of  comparatively 
modern  origin,  as  very  many  of  them  are  dependent  as 
parasites  upon  various  flowering  plants,  often  being  con- 
fined to  a  single  species  as  host,1  and  presumably  have 

1  Host  —  the  animal  or  plant  upon  which  a  parasite  lives. 
80 


THE    FUNGI  81 

acquired  their  specific  peculiarities  from  this  associa- 
tion. 

The  structure  of  these  parasites  and  saprophytes  has 
become  so  profoundly  altered  in  consequence  of  their 
peculiar  mode  of  life  that  it  is  exceedingly  difficult  to 
decide  as  to  their  relationship  with  the  green  plants. 
Naturally  all  trace  of  carbon-assimilating  organs  has  dis- 
appeared, and  the  cell-structure  differs  much  from  that 
of  the  Algoe  except  in  a  small  number  of  forms.  The 
reproductive  parts,  too,  as  a  rule  are  very  different  from 
those  of  the  Algse,  and  it  is  difficult  to  see  any  structu- 
ral affinity  between  the  majority  of  the  Fungi  and  any 
green  plants. 

There  are,  however,  a  number  of  fungi  which  show 
unmistakable  resemblances  to  certain  algge,  and  it  is 
probable  that  these  are  really  related  to  the  latter. 
From  this  resemblance  to  algge  they  are  commonly 
known  as  the  Phycomycetes,  or  "  Alga-fungi,"  and  are 
opposed  to  the  "  Mycomycetes,"  or  True  Fungi,  the  latter 
showing  no  certain  affinity  with  the  Algae,  although  it 
is  not  impossible  that  they  may  be  connected  with  them 
through  the  Phycomycetes.  It  must  be  said,  however, 
that  the  whole  question  of  the  origin  and  affinities  of 
the  higher  fungi  is  very  far  from  being  satisfactorily 
settled. 

THE  PHYCOMYCETES 

This  class  embraces  a  considerable  number  of  fungi, 
some  of  which  show  unmistakable  resemblances  to  cer- 
tain algse,  while  the  relationships  of  others  to  any  green 
forms  are  by  no  means  certain.  Of  the  former  class 
may  be  cited  the  water-moulds  (Saprolegniacese)  and 


82 


EVOLUTION  OF  PLANTS 


the  fungi  known  as  white-rusts  and  mildews.  Of  the 
latter  the  potato-fungus,  Phytophthora  infestans,  the 
cause  of  the  destructive  "  potato-rot,"  is  one  of  the  most 
familiar. 

The  water-moulds  (Saprolegniacese),  (Fig.  21,  A,  D) 
are  aquatic  fungi,  either  saprophytes  on  the  dead  bodies 


FIG.  21  (Phycomycetes). —  A,  a  dead  fly  covered  with  a  growth  of  water- 
mould  (Saprolegnia)  ;  B,  a  sporangium  of  Saprolegnia  about,  to  open ; 
C,  a  single  zoospore ;  D,  part  of  a  plant  of  Saprolegnia  with  two  young 
oogonia,  og;  E,  a  filament  of  white-rust  (Cystopus  candidus)  within 
the  tissues  of  the  host-plant,  showing  the  suckers  or  haustoria  (7i)  by 
which  it  absorbs  its  food  from  the  cells  of  the  host ;  F,  conidia,  or  non- 
sexual  spores  of  Cystopus  being  cut  off  from  the  ends  of  the  filaments  ; 
G,  two  germinating  conidia;  H,  a  free  zoospore  wrhich  has  escaped 
from  the  conidium ;  I,  the  oogonium,  off,  with  the  egg,  o,  in  process  of 
fertilization  by  the  tube  sent  into  the  oogonium  from  the  antheridium, 
an. 

of  insects  and  crustaceans,  or  in  a  few  cases,  like  Sapro- 
legnia ferax,  which  is  a  very  destructive  enemy  of  young 
fish,  they  are  true  parasites.  The  latter  species  often 
causes  great  damage  to  young  fish  in  hatcheries. 

These  water-moulds,  and  their  immediate  allies,  closely 
resemble  in  general  structure  such  siphoneous  algse  as 
Vaucheria,  being,  like  the  latter,  made  up  of  branching 
filaments  which  show  no  division  walls,  but  the  proto- 
plasm lining  the  wall  of  the  tubular  filament  having 


THE   FUNGI  83 

numerous  nuclei.  In  the  common  water-moulds  the 
reproductive  cells  are  also  similar  to  those  of  the  Si- 
phonese.  There  are  sporangia  (Fig.  21,  B)  formed  by 
the  cutting  off  of  the  end  of  a  filament,  and  the  proto- 
plasm of  this  sporangium  then  divides  into  a  large 
number  of  biciliate  zob'spores  (C),  which,  on  escaping, 
germinate  promptly  and  form  new  plants. 

The  sexual  organs  of  the  water-moulds  also  recall 
those  of  Vaucheria,  but  the  oogonium  usually  contains 
more  than  a  single  egg-cell,  and  fertilization  is  not 
effected  by  motile  spermatozoids,  but  directly  by  a  tube 
which  is  sent  out  from  the  antheridium  and  penetrates 
the  wall  of  the  oogonium.  Through  this  tube  the  con- 
tents of  the  antheridium  is  transferred  to  the  egg-cell, 
where  by  a  fusion  of  the  nuclei  of  the  two  cells,  fertili- 
zation is  effected.  The  fertilized  egg  thereupon  secretes 
a  thick  wall  and  becomes  transformed  into  a  spore,  as 
in  the  green  algse. 

In  some  species  of  Saprolegnia  the  spores  develop 
without  fecundation,  antheridia  being  entirely  absent. 
There  are  some  interesting  intermediate  conditions 
where  the  antheridium  is  present,  but  is  entirely  func- 
tionless.  This  degeneration  of  the  reproductive  organs 
is  probably  correlated  with  the  parasitic  or  saprophytic 
habits  of  the  plants,  and  is  a  phenomenon  of  frequent 
occurrence  among  the  higher  fungi,  as  we  shall  see, 
where  the  great  majority  show  no  trace  of  any  sexual 
reproduction. 

Still  more  like  the  algse  are  the  species  of  the  rare 
genus  Monoblepharis,  where  fertilization  is  effected  by 
ciliated  spermatozoids.  Another  remarkable  genus, 
Myrioblepharis,  recently  discovered  by  the  American 


84  EVOLUTION   OF   PLANTS 

botanist  Thaxter,  has  large  multiciliate  zoospores  much 
like  those  of  Vaucheria. 

Besides  these  aquatic  alga-like  fungi  there  are  other 
Phy corny cetes  which  are  not  aquatic.  These  may  be 
either  parasites  upon  the  tissues  of  living  plants,  or  they 
may  be  saprophytes  either  upon  animal  or  vegetable  sub- 
stances. Of  the  first  a  good  example  is  the  so-called 
"white-rust"  (Cystopus  Candidas)  which  infests  the 
common  shepherd's  purse,  Capsella,  as  well  as  other 
cruciferous  plants.  The  masses  of  spores  form  con- 
spicuous chalky-white  blisters  upon  the  stem,  leaves,  and 
flowers  of  the  host,  and  the  growth  of  the  fungus  also 
causes  great  enlargement  and  distortion  of  the  parts 
attacked.  The  structure  of  the  fungus  is  much  like 
that  of  the  related  water-moulds,  and  it  betrays  its 
aquatic  ancestry  by  the  formation  of  ciliated  zoospores 
much  like  those  of  the  water-moulds  (Fig.  21,  H). 
These  zoospores  are  formed  when  the  spores  germinate. 

The  fungus  lives  within  the  body  of  the  host  plant, 
occupying  the  intercellular  spaces  and  sending  into  the 
cells  of  the  host  little  suckers  (Fig.  21,  E,  7i)  by  means  of 
which  it  feeds.  Non-sexual  spores  are  formed  in  rows 
cut  off  from  the  free  ends  of  branches,  just  below  the 
epidermis  of  the  host.  The  epidermis  is  pushed  out 
by  the  growth  of  these  chains  of  spores,  forming  the 
blisters  already  referred  to,  and  finally  is  ruptured  and 
the  spores  then  are  shaken  off  as  a  fine  white  powder. 
Under  proper  conditions  of  temperature  and  moisture, 
the  contents  of  these  spores  divide  into  a  number  of 
parts  which  escape  as  biciliate  zoospores.  This  ordi- 
narily takes  place  when  the  leaves  are  wet  with  rain 
or  heavy  dew.  Oogonia  and  antheridia  are  formed  in 


THE   FUNGI  85 

large  numbers  as  the  fungus  grows  older,  but  these  are 
borne  upon  branches  situated  deep  down  within  the 
host.  Fertilization  is  effected  by  the  formation  of  a 
fertilizing  tube  as  in  the  water-moulds  (Fig.  21,  I). 
After  the  ripe  oospore  is  set  free  by  the  decay  of  the 
tissues  of  the  host,  it  germinates  by  forming  zoospores, 
much  as  do  the  non-sexual  spores. 

There  are  now  known  a  number  of  algse  which  are 
more  or  less  parasitic,  and  which  in  their  manner  of  life 
suggest  these  parasitic  Phycomycetes  which  may  very 
well  have  originated  from  similar  algse.  Such  a  parasitic 
alga  is  Phyllosiphon,  which  is  a  genuine  parasite  within 
the  tissues  of  the  leaves  of  a  species  of  Arisarum,  where 
it  causes  considerable  damage.  This  plant  is  a  most 
interesting  example  of  an  alga  on  the  way  to  become 
a  fungus.  It  still  possesses  some  chlorophyll,  but  that 
it  is  a  true  parasite  is  at  once  shown  by  the  injury 
which  it  inflicts  upon  the  host.  A  probably  analogous 
case  among  the  flowering  plants  which  possess  chloro- 
phyll is  seen  in  such  semi-parasites  as  the  mistletoe  and 
Gerardia. 

Of  the  Phycomycetes  which  show  less  evident  rela- 
tionship to  the  algse,  the  most  familiar  are  the  black- 
moulds.  In  these  the  structure  of  the  plant  is  much 
like  those  already  described,  i.e.  a  branching,  but  un- 
divided, tubular  thallus.  The  reproductive  parts  are, 
however,  quite  different,  none  of  the  reproductive  cells 
ever  showing  motion.  The  sexual  cells,  or  gametes, 
are  usually  alike,  and  fertilization  is  effected  by  the 
fusion  of  two  similar  cells  (Fig.  22,  D),  somewhat  as  in 
some  of  the  desmids  and  pond-scums  among  the  algse. 
It  has  been  suggested  that  possibly  the  black-moulds 


86 


EVOLUTION  OF   PLANTS 


(Mucorini)   may  have   originated  from   algae  like  the 
Conjugate,  just  as  the  water-moulds  are  supposed  to 

have  originated 
from  green  forms 
resembling  Vau- 
cheria ;  but  the 
structure  of  the 
thallus  and  the 
non-sexual  spores 
of  the  black- 
moulds  are  so 
very  different 
from  those  of  the 
Conjugatse  that  it 
seems  much  more 
likely  that  the 


FIG.  22  (Phycomycetes).  —  A,  a  plant  of  a  com- 
mon black-mould  (Mucor  stolonifer) ,  with 
groups  of  stalked  sporangia,  .sp,  arising  from 
the  creeping  filament;  r,  rhizoids,  or  root- 
lets; B,  young;  C,  mature  sporangium  seen 
in  optical  section,  showing  the  columella, 
col. ,-  D-F,  successive  stages  in  the  develop- 
ment of  the  sexual  spore  or  zygospore. 


similarity  in  the 
sexual  cells  is 
purely  accidental. 
At  any  rate  these  fungi  and  their  near  relatives  the 
insect-fungi  (Entomophthoracese)  must  be  regarded  as 
much  further  removed  from  the  algae  than  the  water- 
moulds  and  white-rusts  with  their  ciliated  zoospores,  and 
distinct  oogonia  and  antheridia. 


THE  MYCOMYCETES 

Most  of  the  Fungi  differ  in  structure  so  widely  from 
the  green  plants  that  it  is  difficult  to  find  any  points  of 
resemblance.  These  typical  Fungi  or  Mycomycetes 
differ  in  nearly  all  respects  from  other  plants,  both  in 
their  structure  and  their  methods  of  reproduction. 


THE   FUNGI  87 

Some  of  them,  it  is  true,  show  some  resemblance  to  cer- 
tain of  the  Phycomycetes,  and  may  possibly  have  been 
derived  from  them ;  but  most  of  the  group  are  so  differ- 
ent that  any  attempt  to  determine  their  origin  is  little 
more  than  pure  conjecture. 

These  higher  fungi  are  for  the  most  part  made  up  of 
filaments  (hyphse)  which  are  divided  by  transverse 
walls  formed  in  regular  succession  from  the  end  of  the 
filament,  i.e.  the  hyphse  show  a  definite  apical  growth. 
The  body  of  the  fungus  (mycelium)  may  be  an  indefi- 
nite tangled  mass  of  hyphse,  or  the  plant,  at  least  the 
spore-bearing  portion,  may  have  a  definite  form  and  firm 
texture  owing  to  the  compact  interweaving,  and  often 
actual  cohesion,  of  the  hyphse  into  a  firm  tissue,  such  as 
is  encountered  in  the  spore-fruits  of  the  large  fleshy 
fungi,  like  the  mushrooms,  puff-balls,  etc.  Occasionally, 
as  in  the  large  shelf-shaped  fungi  (Polyporus)  and 
many  of  the  so-called  "  black  fungi,"  the  walls  of  the 
hyphse  become  hard  and  woody  in  texture. 

The  Mycomycetes  may  be  either  parasites  or  sapro- 
phytes, and  occur  under  a  very  great  variety  of  condi- 
tions. Owing  to  the  complete  absence  of  sexual 
reproduction  in  many  of  them,  as  well  as  the  develop- 
ment of  several  very  different  types  of  spores,  even  in 
the  same  species,  much  confusion  has  arisen  in  the  at- 
tempts to  classify  them,  as  not  infrequently  the  same 
plant  has  received  several  different  names  based  upon 
different  stages  of  growth.  This  remarkable  polymor- 
phism has  been  the  cause  of  endless  mistakes  in  nomen- 
clature, and  at  present  the  classification  of  the  whole 
group  is  in  a  chaotic  condition. 

The  question  of  sexuality  in  the  higher  fungi  has 


88  EVOLUTION   OF   PLANTS 

been  the  subject  of  much  controversy,  some  investiga- 
tors going  so  far  as  to  deny  that  it  exists  in  any  mem- 
bers of  the  group.  Recent  investigations,  however, 
have  proved  conclusively  that  in  some  of  the  simpler 
forms,  at  least,  not  only  are  there  genuine  sexual  organs 
present,  but  the  actual  fertilization  has  been  demon- 
strated beyond  any  question.  In  these  forms,  of  course, 
a  direct  comparison  can  be  made  of  the  reproductive 
organs  and  the  structures  arising  as  the  result  of  fer- 
tilization ;  but  where  sexuality  has  been  completely  lost, 
which  appears  to  be  the  case  in  most  of  the  larger  forms, 
it  is  often  impossible  to  determine  positively  which  form 
of  spores  represents  properly  the  product  of  fertilization 
in  those  forms  where  fertilization  occurs.  Especially  is 
this  the  case  where  several  sorts  of  spores  are  developed 
in  the  same  plant. 

Where  parasitism  occurs  in  the  Mycomycetes,  it  often 
attains  a  degree  of  specialization  unparalleled  elsewhere 
in  the  vegetable  kingdom,  and  recalls  the  behavior  of  cer- 
tain animal  parasites.  This  peculiarity  consists  in  the 
passing  from  one  host  to  another,  one  form  of  spores 
being  produced  upon  one  host,  another  upon  the  other. 
One  of  the  first  cases  of  this  "hetercecism  "  to  be  studied 
was  that  of  one  of  the  fungi  which  cause  the  rust  of 
wheat.  It  was  observed  that  the  presence  of  barberry 
bushes  in  the  vicinity  of  wheat  fields  was  accompanied 
by  an  unusual  amount  of  rust  upon  the  growing  grain. 
It  was  finally  discovered  that  the  fungus,  which  in  the 
spring  formed  what  were  popularly  called  "  cluster- 
cups  "  upon  the  barberry  leaves,  was  really  only  one 
stage  of  the  same  fungus  which  later,  passing  from  the 
barberry  to  the  wheat,  caused  the  latter  to  rust,  and 


THE   FUNGI 


89 


that  in  order  for  the  plant  to  complete  its  life-cycle,  it 
was  necessary  that  it  should  grow  in  turn  upon  both 
hosts. 

In  the  eastern  United  States  there  is  another  very 
conspicuous  case  of  this  heteroecism  in  the  fungus  caus- 
ing the  enlargements 
on  the  twigs  of  the 
red  cedar  known  as 
"  cedar-apples  "  (Fig. 
23,  A).  These  mor- 
bid growths  are  due 
to  the  attacks  of  a 
fungus  (Gymnospo- 
rangium)  related  to 
the  wheat-rust,  and 
in  the  spring  the 
large  orange-colored 
masses  of  spores  are 
exceedingly  conspic- 
uous, especially  after 
a  rain,  when  the 
gelatinous  mass  in 
which  they  are  envel- 
oped swells  up.  The 
spores  in  these  masses 
(B,  C)  give  rise  on 
germination  to  sec- 
ondary spores  which 
germinate  at  once  in 
case  they  fall  upon 
the  young  leaves  of  the  wild  crab-apple  or  hawthorn, 
but  will  not  grow  upon  the  cedar.  The  fungus  pro- 


FIG.  23  ( JScidiomycetes) .  —  A,  a  branch 
of  red  cedar  attacked  by  a  parasitic 
fungus  (Gymnosporangium),  forming 
the  excrescence  known  as  a  "cedar- 
apple";  sp,  masses  of  spores  growing 
out  from  the  surface  of  the  cedar-ap- 
ple; B,  two  spores  of  Gymnosporan- 
gium, one  of  which  is  beginning  to 
germinate ;  pr,  the  young  promycelium ; 
C,  a  germinating  spore  which  has  given 
rise  to  a  promycelium  from  each  cell; 
the  secondary  spores,  x,  produced  upon 
the  promycelium  do  not  germinate  upon 
the  cedar,  but  produce  upon  the  haw- 
thorn the  so-called  "secidia,"  or  clus- 
ter-cups; D,  a  leaf  of  cockspur  thorn, 
with  two  groups  of  cluster-cups,  ae ; 
E,  section  through  an  fecidium  of 
another  rust  (Uromyces  caladii). 


90  EVOLUTION  OF  PLANTS 

duced  from  these  spores  upon  the  thorn  (D)  is  abso- 
lutely different  in  appearance  from  that  upon  the  cedar, 
the  spores  being  very  much  smaller  and  produced  in 
chains  within  curious  cup-shaped  receptacles,  much  like 
the  barberry  cluster-cups.  These  spores  on  being  car- 
ried back  to  the  cedar  produce  upon  it  the  form  which 
gives  rise  to  the  cedar-apples.  This  change  of  host  in 
these  parasites  is  exactly  paralleled  by  the  life-history 
of  such  animal  parasites  as  Trichina  and  the  tapeworms, 
which  also  require  more  than  one  host  for  their  com- 
plete development. 

The  more  familiar  of  the  larger  fungi,  such  as  toad- 
stools and  puff-balls,  are  for  the  most  part  saprophytes, 
the  vegetative  portion,  or  mycelium,  being  buried  in 
the  substratum  consisting  of  vegetable  mould  or  earth 
rich  in  organic  matter,  where  it  feeds  and  grows,  and 
finally  sends  up  the  spore-bearing  fruit  (spore-fruit, 
sporophore),  which  is  the  familiar  toadstool  or  puff-ball, 
ordinarily  supposed  to  be  the  whole  plant. 

The  Mycomycetes  (apart  from  the  lichens)  may  be 
arranged  in  three  orders,  which,  however,  show  but 
little  in  common.  These  are  the  Sac-fungi  (Ascomyce- 
tes)  ;  the  Mushrooms  and  their  allies  (Basidiomycetes) ; 
and  Rusts  (JEcidiomycetes). 

THE  ASCOMYCETES 

The  distinguishing  mark  of  this  order  is  the  produc- 
tion of  spores  in  sac-shaped  cells  or  asci,  whence  the 
name.  In  the  lowest  of  the  series,  such  as  the  fungus 
which  causes  the  distortion  of  peach  leaves  known  as 
"curl,"  the  spore-sacs  are  formed  without  any  definite 


THE   FUNGI 


91 


arrangement;  but  in  all  the  higher  ones  they  are  borne  in 
definite  spore-fruits  of  characteristic  form.     This  spore- 
fruit  is  undoubtedly,  in  many  instances,  the  result  of 
fertilization,      being     pro- 
duced by  the  formation  of 
a  peculiar  cell,  the  archi- 
carp, which  corresponds  to 
the  oogonium  of  the  Phy- 
comycetes.    This  is  usually 
fertilized  by  direct  contact 
with  the  antheridium,  and 
from  it,   more   or  less  di- 
rectly,  are    produced    the 
spore-sacs  or  asci. 

A  good  example  of  these 
simpler  Ascomycetes  is 
offered  by  the  mildews 
which  infest  many  plants, 
e.g.  Sphserotheca,  the  com- 
mon rose  mildew.  These 
are  true  parasites,  but  grow 
entirely  upon  the  surface  of 
the  host,  into  whose  epider- 
mal cells  are  sent  suckers 
by  means  of  which  the 
parasite  obtains  nutriment 
from  the  host.  The  myce- 
lium of  the  fungus  sends 
up  vertical  branches  from 

which  are  successively  cut  off  oval  cells  —  spores  or 
"  conidia,"  -  —  which  germinate  promptly  and  through 
whose  means  the  fungus  may  spread  rapidly. 


FIG.  24  (Ascomycetes) .  —  A,  a  chain 
of  conidia  or  non-sexual  spores  of 
a  mildew  (Sphserotheca) ,  one  of 
the  simpler  sac-fungi,  or  Ascomy- 
cetes, growing  upon  the  leaves  of 
the  dandelion ;  B,  the  sexual  repro- 
ductive organs,  archicarp,  ar,  and 
antheridium,  an ;  as  the  result  of 
the  fusion  of  these  there  is  formed 
the  spore-fruit,  C,  containing  the 
single  spore-sac,  or  ascus,  sp,  which 
is  derived  directly  from  the  fertil- 
izer archicarp;  D,  the  ripe  spore- 
fruit  seen  from  without;  E,  a 
single  spore-sac  containing  eight 
spores;  F,  a  cup-fungus  (Ascobo- 
lus) ;  G,  section  of  the  spore-fruit 
of  Ascobolus  showing  the  numer- 
ous spore-sacs,  which  are  also 
derived  from  a  fertilized  archicarp ; 
H,  a  single  ascus  of  Ascobolus. 


92  EVOLUTION   OF   PLANTS 

The  spore-fruit  in  these  mildews  is  very  simple,  and 
in  most  cases  is  preceded  by  the  formation  of  an  archi- 
carp  and  antheridium  (Fig.  24,  B),  both  of  which  are 
simple  cells  cut  off  from  special  branches.  These 
organs  unite,  and  the  contents,  including  the  nuclei, 
fuse,  and  thus  a  true  fertilization  is  effected,  much  as 
in  the  white  rust  described  under  the  Phycomycetes. 
In  the  simplest  of  the  mildews  the  fertilized  archicarp 
divides  into  a  few  cells,  one  of  which  grows  directly  into 
an  oval  sac  in  which,  after  a  preliminary  division  of  the 
nucleus  into  eight,  there  is  formed  about  each  of  these 
nuclei  an  aggregation  of  protoplasm  which  becomes  sur- 
rounded by  a  cell-wall  and  forms  a  spore.  There  are 
still  simpler  Ascomycetes  where  the  fertilized  archicarp 
becomes  at  once  transformed  into  the  ascus.  From  the 
filaments  close  to  the  archicarp  there  grow  out  a  num- 
ber of  short  branches  which  form  a  compact  covering 
about  the  asci,  the  whole  structure  forming  the  "  peri- 
thecium  "  or  spore-fruit  of  the  mildew.  In  many  of  the 
mildews  the  cells  forming  the  wall  of  the  perithecium 
develop  hair-like  appendages  of  curious  and  characteris- 
tic shapes,  which  constitute  one  of  the  best  means  of  dis- 
tinguishing the  different  genera  and  species. 

Closely  related  to  the  mildews  is  the  common  blue- 
mould,  Penicillium,  and  the  herbarium-mould,  Euro- 
tium.  These  are  saprophytes,  and  the  spores  are  borne 
on  branching  conidiophcres  instead  of  in  simple  chains. 

The  spore-fruit  of  some  of  the  larger  Ascomycetes 
is  very  conspicuous,  and  in  the  case  of  the  pretty  cup- 
fungi  of  various  vivid  colors,  scarlet,  orange,  yellow,  etc. 
These  large  spore-fruits  are  usually  the  product  of  a 
number  of  archicarps,  i.e.  they  are  compound  in  nature, 


THE   FUNGI  93 

but  in  many  cases  they  seem  to  arise  in  a  purely  vege- 
tative way  without  any  formation  of  sexual  organs. 

Many  of  the  Ascomycetes  show  remarkable  polymor- 
phism, this  being  especially  marked  in  the  "  Black- 
fungi  "  or  Pyrenomycetes,  of  which  the  ergot  of  the 
rye  and  the  "black  knot"  of  cherries  and  plums  are 
examples.  In  these  there  are,  in  addition  to  the  asco- 
spores,  several  different  forms  of  conidia  cut  off  from 
the  tips  of  the  hyphse. 

THE  BASIDIOMYCETES 

The  Basidiomycetes  include  most  of  the  more  familiar 
large  fungi,  known  popularly  as  mushrooms,  toadstools, 
puff-balls,  etc.  These  fungi  have  no  very  evident  affin- 
ity with  the  sac-fungi,  and  as  yet  none  of  them  have 
shown  any  traces  of  sexual  reproduction.  The  great 
majority  are  saprophytes,  the  mycelium  or  vegetative 
filaments  ramifying  extensively  through  the  substratum, 
which  usually  is  earth  rich  in  decaying  vegetation,  rot- 
ten wood,  or  similar  dead  organic  matter.  From  this 
mycelium  the  spore-fruit  arises,  apparently  in  all  cases 
as  a  purely  vegetative  growth.  Most  Basidiomycetes 
produce  but  one  kind  of  spores,  borne  upon  club-shaped 
cells  known  as  basidia  (Fig.  25,  E),  and  it  is  still  an 
open  question  whether  the  spore-fruit  in  these  can  prop- 
erly be  compared  to  that  of  the  Ascomycetes. 

The  basidia  are  swollen,  club-shaped  cells,  borne  at 
the  end  of  hyphse,  and  from  each  basidium  grow  out 
several,  usually  two  or  four,  little  protuberances,  each 
of  which  produces  a  single  spore  at  the  tip.  These 
basidia  are  usually  formed  upon  special  parts  of  the 


94 


EVOLUTION  OF  PLANTS 


spore-fruit,  which  may  have  a  definite  character,  as  in 
the  mushroom,  where  the  "  gills "  (Fig.  25,  A,  #)  .are 
of  this  nature.  The  spores  on  germination  form  a 
new  mycelium,  which  in  time  produces  spore-fruits. 


FIG.  25  (Basidiomycetes).  —  A,  a  cluster  of  spore-fruits  of  the  common  mush- 
room, arising  non-sexually  from  the  mycelium,  »/,  which  is  buried  in 
the  ground :  B,  a  very  young  mushroom ;  C,  a  section  of  an  older  one 
showing  the  gills,  g,  upon  which  the  spores  are  borne ;  D,  diagram 
showing  a  section  of  a  gill  with  the  spore-bearing  "  basidia,"  b,  cover- 
ing its  surface;  E,  i,  young,  n,  mature  basidium  of  a  toadstool  (Co- 
prinus) ,  showing  the  spores  borne  at  the  summit ;  F,  spore-fruit  of  Tre- 
mella,  one  of  the  lower  Basidiomycetes;  the  spores  cover  the  whole 
surface  of  the  irregular  spore-fruit;  G,  a  bird's-nest  fungus  (Cyathus) : 
the  spores  are  borne  inside  the  "sporangia,"  sp,  within  the  cup:  H, 
earth-star  (Geaster),  one  of  the  Gasteromycetes  allied  to  the  puff-balls. 
(Figs.  A,  B,  after  Warming;  C,  after  Atkinson.) 

The  lowest  of  the  Basidiomycetes  show  analogies 
with  the  rusts  (^Ecidiomycetes),  and  do  not  have  the 
basidia  restricted  to  any  definite  part  of  the  spore-fruit, 
but  they  may  be  produced  all  over  it,  as  in  the  soft 
gelatinous  Tremella  (Fig.  25,  F),  whose  convoluted 
soft  yellow  or  orange  masses  are  not  uncommon  on 


THE   FUNGI  95 

rotten  twigs  or  stumps.  In  all  of  the  higher  ones, 
however,  the  "hymenium"  or  spore-bearing  areas  are 
restricted  to  definite  portions  of  the  spore-fruit. 

In  the  highest  group  of  all,  represented  by  the  puff- 
balls  (Lycoperdon)  and  their  allies,  the  spores  are 
borne  within  the  spore-fruit,  and  are  only  exposed 
when  they  are  perfectly  ripe. 

THE  jEciDIOMYCETES 

Under  the  name  of  ^Ecidiomycetes  are  included  the 
parasitic  fungi,  known  popularly  as  "rusts"  and 
"  smuts,"  which  are  among  the  most  destructive  of 
plant  parasites.  The  various  forms  of  wheat-rust,  and 
the  corn-smut,  are  familiar  examples  of  this  class.  Both 
of  these  orders  show  certain  analogies  with  the  lower 
Basidiomycetes,  and  are  possibly  related  to  them.  As 
in  the  latter,  no  trace  of  sexual  organs  has  yet  been 
discovered.  Unlike  the  Basidiomycetes  they  often 
show  a  remarkable  tendency  to  polymorphism,  which 
reaches  its  most  marked  development  in  the  rusts, 
where,  as  we  have  seen,  in  the  wheat-rust  and  cedar- 
rust,  it  is  complicated  by  the  habit  of  hetercecism,  or 
the  passing  from  one  host  to  another  in  the  course  of 
development.  Owing  to  the  absence  of  sexual  organs, 
the  same  difficulty  is  experienced  here  as  in  the  Basidio- 
mycetes, of  deciding  which  form  corresponds  to  the 
spore-fruit  in  the  Ascomycetes  when  this  is  developed 
as  the  result  of  fertilization. 

While  some  of  the  rusts  resemble  the  lower  Basidio- 
mycetes, the  smuts  show  certain  analogies  with  the 
Phycomycetes,  but  it  is  doubtful  if  the  latter  resem- 


96  EVOLUTION   OF  PLANTS 

blances  indicate  any  real  relationship,  and  it  is  quite  as 
likely  that  they  are  signs  of  degeneration,  the  most 
marked  resemblance  being  the  absence  of  division  walls 
in  the  hyphse.  On  the  whole,  the  smuts  seem  to  be  really 
much  more  nearly  related  to  the  rusts. 

We  have  finally  to  consider,  among  the  Fungi,  the 
peculiar  organisms,  the  yeast-plants,  which  are  the 
principal  agents  in  alcoholic  fermentation.  They  are 
unicellular  forms,  the  individual  cells  being  oval  in 
shape  and  multiplying  rapidly  by  the  peculiar  mode  of 
cell-division  known  as  budding.  They  may  also, 
under  special  conditions,  develop  a  number  of  spores  by 
internal  division.  The  structure  of  the  cells  is  ex- 
tremely simple,  and  the  presence  of  a  definite  nucleus  is 
still  open  to  question. 

The  relation  of  the  yeast-fungi  to  the  other  members  of 
the  group  is  still  a  matter  of  controversy.  Some  authori- 
ties consider  them  to  be  very  low  organisms,  having 
some  affinity  with  the  bacteria;  others,  on  the  strength  of 
their  forming  spores  internally,  somewhat  like  the  Asco- 
mycetes,  regard  them  as  the  lowest  members  of  this 
group;  still  others  have  thought  that  they  represent  a 
permanent  conidial  stage  of  forms  related  to  the  smuts, 
as  in  the  latter  the  spores  under  certain  conditions  may 
bud  much  as  do  the  yeast-cells :  that  is,  the  yeast- 
cells  are  supposed  to  be  spores  arrested  in  their  devel- 
opment so  that  they  never  form  filaments  or  hyphse. 
Which  of  these  hypotheses  is  the  correct  one,  must  at 
present  be  left  unanswered. 


THE   FUNGI  97 

THE  LICHENS 

In  connection  with  the  true  Fungi  there  must  be  con- 
sidered the  Lichens.  While  there  is  no  doubt  that  these 
are  sufficiently  distinct  to  form  a  separate  class,  never- 
theless their  obvious  relationship  to  other  fungi,  mostly 
Ascomycetes,  forbids  the  establishment  of  a  subkingdom 
coordinate  with  Algse  and  Fungi. 

These  curious  organisms  exhibit  a  remarkable  type 
of  parasitism,  or  perhaps  better,  symbiosis,  where  two 
plants,  an  alga  and  a  fungus,  are  so  intimately  associ- 
ated as  practically  to  form  a  single  plant.  An  exami- 
nation of  the  thallus  of  a  lichen  shows  it  to  be  made 
up  of  densely  woven  and  more  or  less  coherent  hyphse, 
among  which  are  found  numerous  green  cells.  The 
latter  are  usually  aggregated  near  the  outside  of  the 
thallus,  and  a  careful  examination  shows  them  to  be 
certain  low  algae,  which  can  be  readily  identified  as 
species  often  growing  quite  apart  from  the  lichen. 
If  these  algse  are  isolated,  and  given  proper  conditions 
for  growth,  they  flourish  perfectly,  showing  that  they 
are  not,  in  any  true  sense  of  the  word,  really  dependent 
upon  the  lichen  for  their  existence.  While  in  some 
respects  the  hyphee  of  most  lichens  differ  somewhat 
from  those  of  other  fungi,  still  the  general  structure  is 
very  similar,  and  the  fructification  corresponds  exactly 
with  that  of  typical  fungi,  especially  the  Ascomycetes, 
to  which  most  lichens  are  undoubtedly  related.  A  few, 
however,  are  Basidiomycetes. 

It  was  supposed  by  the  earlier  students  of  the  lichens 
that  the  green  cells  were  direct  outgrowths  of  the  color- 
less hyphse,  but  the  more  careful  investigations  of  later 


98  EVOLUTION   OF  PLANTS 

years  have  proved  conclusively  that  the  lichen  thallus 
is  a  compound  organism,  consisting  usually  of  an  asco- 
mycetous  fungus  parasitic  upon  an  alga.  While  the 
alga  probably  derives  some  benefit  from  this  association 
with  the  fungus,  and  by  the  shelter  afforded  by  the 
fungus  can  probably  grow  where  otherwise  it  could  not, 
still  the  advantage  is  much  more  on  the  side  of  the 
fungus,  which  without  the  alga  is  incapable  of  growth 
and  soon  perishes.  Through  this  peculiar  form  of 
parasitism  the  fungi  have  become  decidedly  altered,  so 
that  they  differ  very  considerably  from  any  other  As- 
comycetes;  but  the  algse  are  identical,  even  to  the 
species,  with  forms  which  live  quite  free  from  the 
lichen.  The  germinating  spores  of  a  lichen  produce  a 
mass  of  colorless  filaments,  like  those  of  other  fungi, 
and  if  these  come  in  contact  with  the  proper  algal  form, 
they  will  attach  themselves,  and  in  time  the  fully 
developed  lichen  thallus  is  produced.  If,  howe"ver,  no 
algal  cells  are  within  reach,  the  mycelium  soon  dies 
unless  supplied  artificially  with  carbonaceous  food. 

The  lichens,  no  doubt,  represent  a  very  specialized 
group  of  plants,  but  they  cannot  properly  be  separated 
from  the  Fungi,  as  they  are  so  obviously  related  to 
them,  and  it  is  the  fungus  element  of  the  lichen  which 
is  the  predominant  one.  Moreover,  not  all  the  lichens 
are  related  among  themselves,  as  it  is  perfectly  evident 
that  this  peculiar  form  of  parasitism  has  arisen  quite 
independently  in  different  groups  of  the  Ascomycetes, 
as  well  as  in  the  Basidiomycetes.  The  algal  elements 
found  in  lichens  belong  also  to  a  number  of  widely 
separated  groups,  e.g.  Protococcacese,  Cyanophycese, 
Confervacese. 


-UNIVERSITY 
THE  FUNG  99 


SUMMARY 

The  Fungi  as  a  whole  must  be  considered  as  having 
but  slight  affinities  with  the  green  plants.  While  the 
Phycomycetes  or  Alga-fungi  show  undoubted  resem- 
blances to  certain  green  algse,  especially  the  Siphonese, 
even  here  there  are  marked  differences,  although  not  so 
great  but  that  a  possible  derivation  of  the  former  from 
green  ancestors  is  conceivable.  The  Phycomycetes  are 
not,  however,  to  be  considered  as  a  homogeneous  class, 
but  rather  as  an  assemblage  of  chlorophylless  plants 
derived  independently  from  diverse  green  ancestors,  in 
much  the  same  way  that  various  colorless  parasites  and 
saprophytes  among  the  flowering  plants  have  arisen 
independently. 

While  the  question  of  the  origin  of  the  Phycomycetes 
is  fairly  clear,  this  is  by  no  means  the  case  with  the 
much  more  numerous  and  varied  Mycomycetes,  or  true 
Fungi.  It  is  true  that  there  are  certain  points  of 
similarity  between  the  lower  Ascomycetes  and  the  Phy- 
comycetes, and  the  smuts  also  recall  in  some  respects 
the  latter;  but  it  is  by  no  means  universally  admitted 
that  such  a  connection  does  really  exist,  and  the  origin 
of  the  Mycomycetes  must  for  the  present  be  considered 
as  at  least  doubtful.1 

Moreover,  the  interrelationships  of  the  Mycomycetes 
are  very  obscure.  The  complete  lack  of  sexuality  in 
so  many  of  them  makes  a  determination  of  the  ho- 
mologies  in  their  structure  exceedingly  difficult;  and  as 

1  One  very  peculiar  family  of  Ascomycetes,  the  Laboulbeniacese, 
which  are  parasites  in  insects,  show  many  analogies  with  the  red  algse, 
and  may  possibly  have  been  derived  from  them. 


100  EVOLUTION   OF   PLANTS 

the  groups  now  exist,  the  three  classes,  Ascomycetes, 
JEcidiomycetes,  and  Basidiomycetes,  have  very  little 
in  common,  although  it  is  probable  that  the  two  latter 
have  had  a  common  origin. 

The  Lichens  must  be  supposed  to  have  had  a  multiple 
origin,  like  the  Phycomycetes.  The  majority  have  been 
derived  from  different  groups  of  ascomycetous  fungi, 
but  some  of  them  are  allied  to  the  Basidiomycetes. 

We  may  then  consider  the  Fungi  as  an  aberrant 
group  of  plants,  probably  —  but  not  certainly — derived 
from  originally  green  ancestors,  but  which  have  di- 
verged so  widely  from  the  parent  stock  that  they  have 
lost  nearly  all  of  their  original  characteristics. 


CHAPTER   VI 

MOSSES  AND  LIVERWORTS    (BEYOPHYTA} 

THE  Fungi  form,  as  we  have  seen,  an  aberrant  as- 
semblage of  plants,  probably  derived  from  green  ances- 
tors, but  not  giving  rise  to  any  higher  forms.  In 
seeking  for  the  point  of  connection  between  the  higher 
green  plants  and  the  Thallophytes,  we  must  look  then 
to  the  Algse,  and  the  forms  among  these  which  show 
the  most  evident  relationship  with  the  lower  terrestrial 
green  plants  are  the  Green  Algse,  or  Chlorophyceae. 

While  the  Algae  are  practically  all  aquatics,  the 
plants  we  are  now  to  consider  are  for  the  most  part 
terrestrial.  The  lowest  of  these  are  the  Bryophytes  or 
Mosses,  using  this  term  in  its  broadest  sense.  These 
are  readily  divisible  into  two  classes,  the  Liverworts,  or 
Hepaticae,  and  the  true  Mosses,  Musci.  Of  these  the 
former  show  the  most  evident  resemblances  to  the  Algae, 
and  will  be  considered  first. 

These  plants  are  usually  moisture-loving  forms,  a  few 
being  actually  aquatic,  but  many  of  them  are  so  con- 
stituted that  they  may  be  completely  dried  up  without 
injury,  quickly  reviving  when  supplied  with  moisture. 

The  lowest  liverworts  (Fig.  27,  A,  C)  are  little  flat 
green  plants  of  very  simple  structure,  and  may  be 
readily  compared  to  some  of  the  green  algae,  such  as 
Coleochaete.  However,  when  the  reproductive  parts 

101 


102  EVOLUTION  OF  PLANTS 

are  examined,  it  is  seen  that  even  the  lowest  mosses 
are  far  more  complicated  than  any  of  these  algse. 

The  zoospores,  or  motile  non-sexual  reproductive  cells 
of  the  algse,  are  wanting  completely  in  the  mosses,  but 
among  the  lowest  liverworts  there  have  been  discovered 
certain  cells  which  perhaps  represent  them.  In  these 
forms  the  contents  of  an  ordinary  thallus  cell  are 
ejected  in  the  form  of  a  unicellular  or  two-celled  body 
very  much  like  the  zoospores  of  many  algse,  but  desti- 
tute of  cilia.  The  method  of  development  of  these 
bodies  suggests  that  in  them  we  have  the  last  trace  of 
zoospore  formation,  the  absence  of  cilia  being  corre- 
lated with  the  terrestrial  habit  of  the  liverworts.  Spe- 
cial non-sexual  reproductive  bodies  (buds  or  gemmse) 
of  an  entirely  different  kind  are  not  uncommon  in 
many  of  the  higher  forms,  both  among  the  Hepaticee 
and  the  true  mosses. 

The  lower  Hepaticse  are  of  especial  importance  in  a 
study  of  the  origin  of  the  higher  plants,  as  there  is  good 
reason  to  believe  that  they  represent  the  most  primitive 
of  existing  chlorophyll-bearing  terrestrial  plants,  and 
probably  have  given  rise  to  all  the  higher  types  of 
vegetation. 

The  liverworts,  in  common  with  the  other  mosses  and 
the  ferns,  have  the  egg-cell  borne  in  a  peculiar  organ,  of 
very  uniform  structure  in  all  of  them,  known  as  the 
archegonium  (Fig.  26,  A,  B)  ;  and  on  account  of  this 
uniformity  of  structure,  mosses  and  ferns  together  are 
often  united  into  one  great  division,  the  Archegoniafoe. 
The  archegonium  usually  has  the  form  of  a  long- 
necked  flask  in  whose  enlarged  base,  or  venter,  is  found 
the  egg-cell.  The  nearest  approach  to  this  structure 


MOSSES   AND   LIVERWORTS 


103 


among  the  algse  is  found  in  the  stoneworts  (Characese), 

but  the  differences  in  the  vegetative  parts  between  these 

and  the  Hepaticse  are  too  great  to  admit  of  the  idea  of 

any  but  the  remotest 

relationship    existing         A    rCx        B 

between  the  two,  and 

at  present  it  must  be 

admitted     that     the 

gulf    between    Algse 

and  Archegoniates  is 

a  very  deep  one. 

The  antheridium  is 
not  so  different  from 
that  of  some  algse,  but 
is  much  more  com- 
plicated than  in  any 
but  the  Characese.  In 
the  Archegoniates  it 
has  the  form  of  a 
capsule  (Fig.  26,  C), 
which  in  the  lower 

forms  is  usually  stalked.  The  central  part  is  divided 
into  many  small  cells,  in  each  of  which  is  developed 
a  spermatozoid.  The  latter  is  very  much  like  those  of 
most  algse,  and  like  them  is  provided  with  cilia  (Fig. 
26,  D). 

Throughout  the  whole  group  of  the  Archegoniates 
water  is  necessary  for  the  opening  of  both  archegonium 
and  antheridium,  the  water  swelling  up  the  mucilagi- 
nous cell-walls  of  the  interior  of  the  organs,  thus  forcing 
them  open.  The  liberated  spermatozoids  then  swim  to 
the  open  archegonium,  which  in  the  mean  time  has  dis- 


FIG.  26.  —  A,  longitudinal  section  of  the 
archegonium  of  a  liverwort  (Targionia), 
showing  the  central  row  of  cells ;  B,  a 
similar  section  of  the  ripe  archegonium 
of  Riccia ;  the  cells  of  the  axial  row  are 
disorganized  and  the  egg,  o,  lies  free  in 
the  enlarged  venter  of  the  archegonium  ; 
C,  longitudinal  section  of  the  antherid- 
ium of  Riccia,  showing  the  mass  of  sperm- 
cells  surrounded  by  a  single  layer  of 
peripheral  cells ;  D,  a  free  spermatozoid 
of  Fimbriaria  Californica. 


104  EVOLUTION   OF  PLANTS 

charged  the  disintegrated  cells  of  the  canal  traversing 
the  neck,  and  thus  cleared  the  passage  to  the  egg-cell 
within  the  venter.  The  spermatozoids  enter  the  open 
archegonium  and  make  their  way  to  the  central  cell, 
where  one  of  them  penetrates  the  egg-cell,  thus  effect- 
ing its  fertilization. 

The  necessity  of  water  for  the  effecting  of  fertiliza- 
tion is  significant,  as  it  would  seem  to  be  a  reversion 
to  the  aquatic  condition  of  the  algal  ancestors  of  the 
Archegoniates. 

ALTERNATION  OF  GENERATIONS 

The  alternation  of  sexual  and  non-sexual  individuals  is 
met  with  in  many  algse,  but  there  is  usually  little  differ- 
ence in  the  structure  of  the  two,  aside  from  the  repro- 
ductive organs.  Thus  in  CEdogonium,  or  Vaucheria, 
there  is  no  apparent  difference  between  the  plants  which 
produce  zoospores  and  those  which  bear  the  sexual  cells ; 
and  sometimes,  at  least,  the  formation  of  one  sort  of 
reproductive  cells  or  the  other  is  entirely  a  question  of 
nutrition. 

In  the  higher  Chlorophycese,  and  this  is  suggested  in 
CEdogonium,  it  will  be  remembered  that  the  spore  pro- 
duced as  the  result  of  fertilization  does  not  at  once  grow 
into  a  plant  like  the  parent,  but  there  is  first  a  division 
of  its  contents  into  four  zoospores  which  give  rise  to  as 
many  new  individuals.  In  Coleochsete  (see  Fig.  10), 
the  genus  which  on  the  whole  approaches  most  nearly 
to  the  lower  Archegoniatse,  the  germinating  resting- 
spore  produces  a  multicellular  body,  from  each  of  whose 
cells  a  zoospore  is  produced  which  then  develops  into 
the  new  plant. 


MOSSES   AND   LIVERWORTS  105 

In  the  Archegoniates  the  structure  arising  from  the 
fertilized  egg  is  much  more  complicated  than  in  any  of 
the  algae.  Here,  also,  the  egg  after  fertilization  secretes 
a  cell-wall  about  itself,  but  instead  of  remaining  at  rest 
for  a  long  time,  growth  begins  almost  at  once.  The 
plant  thus  formed  is .  entirely  different  from  the  one 
which  produces  the  sexual  organs,  and  the  reproductive 
cells  to  which  it  gives  rise  differ  entirely  from  those  of 
the  sexual  plant.  These  cells  are  purely  non-sexual  in 
character  and  capable  of  germinating  at  once.  They 
are  spores  which  differ  from  the  corresponding  ones  of 
the  green  algae  in  being  destitute  of  cilia  and  provided 
with  a  very  firm  membrane  which  enables  them  to  resist 
extremes  of  temperature  and  dryness. 

The  spores  in  all  the  Archegoniates  are  formed  in 
groups  of  four  from  the  division  of  a  common  mother- 
cell.  The  tissue  from  which  the  sporogenous  cells  arise 
is  termed  the  "  archesporium."  These  spores  on  germi- 
nation give  rise,  not  to  another  spore-bearing  plant, 
but  to  the  sexual  one.  This  alternation  of  sexual  and 
non-sexual  individuals  is  a  constant  characteristic  of  the 
Archegoniates,  and  the  two  phases  are  known  respec- 
tively as  the  gametophyte  (sexual)  and  sporophyte 
(non-sexual),  —  convenient  terms  which  will  be  adopted 
in  the  future  discussion  of  the  group. 

Among  the  lower  Archegoniates,  as  in  the  algae,  it 
is  the  gametophyte  which  is  predominant  and  the  spo- 
rophyte is  small  and  inconspicuous,  looking  like  a  mere 
appendage  of  the  gametophyte ;  but  as  we  ascend,  we 
shall  see  how  the  gametophyte  becomes  more  and  more 
subordinated  to  the  sporophyte,  which  finally  becomes 
an  independent  long-lived  plant,  while  the  gametophyte 


106 


EVOLUTION  OF  PLANTS 


simply  lives  long  enough  to  produce  the  sexual  organs 
and  to  nourish  the  embryo-sporophyte  until  it  becomes 
self-supporting. 


THE  LIVERWORTS   (Hepaticce) 

Like  the  Confervacese  among  the  green  algse,  the 
Liverworts  seem  to  represent  a  low  generalized  assem- 
blage of  plants  showing 
affinities  with  several 
other  groups,  and,  in- 
deed, they  probably  rep- 
resent the  ancestral  forms 
from  which  have  arisen 
all  the  higher  plants. 
While  the  lower  Hepaticse 
are  but  little  niQre  com- 
plicated than  some  of  the 
Confervacese,  others  show 
a  considerable  degree  of 
differentiation  of  the  ga- 
me tophyte.  The  latter, 
in  the  simplest  cases  (Fig. 
27,  A,  C),  is  a  small  flat 
body  or  thallus  composed 
of  almost  uniform  green 
cells,  the  whole  fastened 
to  the  ground  by  numerous 


FIG.  27  (Hepaticse) .  —  A,  B,  C,  thal- 
lose  liverworts  ;  A,  Riccia,  sp,  the 
very  small  sporophyte  ;  B,  Cono- 
cephalus,  st,  stomata;  C,  Metz- 
geria ;  D,  Blasia,  a  liverwort  which 
shows  the  first  formation  of  leaf- 
like  organs,  I ;  E,  Lejeunia,  a  foli- 
ose  liverwort  with  definite  stem 
and  three  rows  of  leaves,  large 
dorsal  ones,  and  small  ventral 
ones,  am. 


delicate  hairs  or  rhizoids. 

This  thallus  grows  by  the  divisions  of  a  definite  apical 
cell,  which  differs  in  different  genera,  or  even  in  differ- 
ent species  of  the  same  genus. 


MOSSES   AND   LIVERWORTS  107 

Starting  from  this  simple  type,  the  development  of 
the  gametophjte  has  proceeded  in  several  directions, 
two  of  which  are  specially  noteworthy.  In  the  first 
place,  while  the  gametophyte  has  retained  its  primitive 
thallose  form,  there  has  been  a  very  considerable 
amount  of  differentiation  in  the  tissues,  which  are 
divided  into  a  dorsal  region,  mainly  occupied  by  an 
elaborate  system  of  assimilating  tissues,  and  a  ventral 
mass  of  colorless  cells.  The  assimilative  apparatus  in 
the  most  highly  specialized  forms  consists  of  a  series  of 
large  chambers  into  which  the  chlorophyll-bearing  cells 
project,  which  communicate  with  the  outside  atmosphere 
by  means  of  curious  pores  which  may  be  compared  func- 
tionally at  least  with  the  stomata  of  the  higher  plants 
(Fig.  27,  B).  The  rhizoids  are  also  peculiarly  modi- 
fied, and  scales  are  developed  from  the  ventral  surface 
of  the  thallus.  In  the  higher  members  of  this  group 
(Marchantiacese),  the  sexual  organs  are  borne  upon 
modified  branches,  and  in  some  cases  peculiar  non-sexual 
reproductive  bodies,  gemmae,  are  produced  in  special 
receptacles. 

The  second  type  of  differentiation  is  shown  by  the 
foliose  or  leafy  Hepaticse,  the  "scale-mosses."  These 
comprise  much  the  greater  part  of  the  existing  liver- 
worts, and  are  distinguished  from  the  lower  forms  by 
having  a  distinct  axis  with  definite  leaves  or  assimila- 
tive organs  (Fig.  27,  E).  Both  stem  and  leaves  are  of 
the  simplest  possible  structure,  all  the  cells  being  alike, 
and  the  leaves  are  composed  of  but  a  single  layer  of 
cells,  but  these  simple  leaves  form  very  efficient  assimi- 
lating organs.  The  scale-mosses  are  much  the  com- 
monest of  liverworts,  and  their  adaptation  to  various 


108  EVOLUTION  OF  PLANTS 

conditions,  as  well  as  their  abundance  and  variety, 
indicate  a  more  modern  type  than  the  thallose  forms 
with  which  they  are  connected  by  various  intermediate 
conditions  (Fig.  27,  D). 

Some  of  the  foliose  Hepaticse,  especially  certain  tropi- 
cal types,  show  extremely  curious  modifications  of  the 
leaves  to  form  reservoirs  of  moisture  or  even  traps  for 
small  Crustacea,  recalling  those  found  in  some  flower- 
ing plants,  such  as  the  bladder-weed  (Utricularia). 

The  range  of  structure  in  the  sporophyte  of  the  He- 
paticse  is  great,  and  a  study  of  the  different  types  is 
most  instructive  in  showing  the  growing  importance  of 
the  sporophyte  in  passing  from  the  lower  forms  to  those 
which  approximate  the  structure  of  the  higher  plants. 

The  simplest  sporophyte  is  met  with  in  the  genus  Ric- 
cia  (Fig.  27,  A,  Fig.  28,  B),  which  comprises  a  number 
of  small  thallose  liverworts,  where  there  is  no  trace 
of  any  differentiation  of  the  gametophyte  into  stem 
and  leaves ;  but  the  thallus  is  not  so  primitive  as  in 
certain  other  forms  which  have  a  more  highly  developed 
sporophyte.  The  sexual  organs  are  borne  upon  the 
dorsal  surface  of  the  gametophyte,  but  not  arranged 
in  any  definite  order.  They  have  the  typical  structure 
found  in  other  Hepaticse.  The  archegonium  (Fig.  26, 
B)  contains  the  egg  in  the  enlarged  ventral  portion, 
and  when  the  plants  are  covered  with  water,  it  opens 
and  allows  the  sperm atozoids,  which  have  at  the  same 
time  been  liberated  from  the  ripe  antheridium,  to  swim 
into  it.  The  spermatozoid  penetrates  the  egg-cell, 
which  thereupon  is  stimulated  into  active  growth,  and 
develops  into  the  sporophyte,  or  sporogonium,  as  it  is 
commonly  termed  in  the  mosses.  The  development  of 


MOSSES   AND  LIVERWORTS 


109 


the  sporophyte  in  Riccia  is  very  simple,  recalling  that 
of  Coleochsete  (Fig.  10,  C)  among  the  algse,  and  there 
is  no  difficulty  in  understanding  how  a  sporophyte  of 
the  type  of  that  in  Riccia  may  have  originated  from 
that  of  Coleochaete. 


FIG.  28  (Development  of  the  sporophyte  in  Hepaticse). — A,  young  em- 
bryo-sporophyte  of  Targionia  ;  i,  u,  the  first  division  walls  in  the  fertil- 
ized egg;  B,  longitudinal  section  of  the  young  sporophyte  of  Riccia,  in- 
cluded within  the  archegonium,  ar;  all  of  the  cells,  except  a  single 
peripheral  layer,  produce  spores  ;  C,  longitudinal  section  of  the  young 
sporophyte  of  Sphterocarpus ;  only  the  upper  part  produces  spores,  the 
lower  half  forming  an  organ  of  absorption,  the  foot,  /;  D,  a  similar 
section  of  the  embryo  of  Anthoceros ;  the  nucleated  cells  represent 
the  archesporium  or  sporogenous  tissue ;  E,  cross-section  of  an  older 
sporophyte  of  Anthoceros,  showing  the  small  amount  of  sporogenous 
tissue,  sp ;  F,  section  through  a  spore-tetrad  of  Fossombronia  lonyiseta; 
only  three  of  the  four  spores  show ;  G,  a  ripe  spore  of  the  same  species ; 
H,  an  elater. 

The  first  result  of  fertilization  is  the  formation  of  a 
cellulose  membrane  about  the  egg,  which  thus  is  trans- 
formed into  a  spore  directly  comparable  to  the  resting- 
spore  of  such  an  alga  as  CEdogonium.  Here,  however, 
instead  of  remaining  at  rest  for  a  long  period,  it  ger- 
minates at  once.  It  first  divides  by  a  transverse  wall 


110  EVOLUTION   OF  PLANTS 

into  equal  parts,  and  this  is  followed  by  two  other 
walls  at  right  angles  to  the  first,  and  the  globular 
"embryo,"  as  it  is  now  called,  is  composed  of  eight 
nearly  equal  cells.  Soon  there  are  formed  a  series  of 
walls  by  which  a  single  layer  of  peripheral  cells  is  sepa- 
rated from  the  central  mass  of  tissue  (Fig.  28,  B),  and 
the  cells  of  the  latter,  after  several  preliminary  divisions, 
separate,  and  each  one  divides  into  four  equal  parts  or 
spores.  This  division  is  preceded  by  a  double  division 
of  the  cell-nucleus,  and  it  is  not  until  the  four  nuclei 
are  complete  that  the  division-walls  arise  between 
them,  by  which  the  sporogenous  cell  is  divided  into 
the  four  tetrahedral  spores.  These  are  at  first  thin 
walled  (F),  but  later  develop  a  thick  membrane  (G), 
and  the  spore  as  it  ripens  becomes  filled  with  oil  and 
other  nutritive  substances.  The  mature  sporophyte  in 
Riccia  is  simply  a  globular  capsule,  completely  filled  with 
a  mass  of  thick-walled  spores.  No  assimilative  tissue 
is  developed  by  the  sporophyte,  and  it  is  entirely  de- 
pendent for  its  subsistence  upon  the  gametophyte.  The 
venter  of  the  archegonium  continues  to  grow  with  the 
enclosed  sporophyte,  and  forms  a  protective  covering 
about  it,  much  as  do  the  enveloping  cells  in  Coleochsete, 
although  in  the  latter  the  protective  cells  are  entirely 
undeveloped  before  fertilization. 

The  mass  of  spores  remains  enclosed  within  the 
archegonium-venter  ("  calyptra  ")  until  they  are  liber- 
ated by  its  decay,  as  the  older  parts  of  the  thallus  die. 
After  a  period  of  rest,  these  spores  germinate  if  they 
are  supplied  with  the  proper  conditions  of  light,  heat, 
and  moisture.  The  spores  give  rise,  not  to  a  sporo- 
phyte, but  to  a  gametophyte,  and  it  is  interesting  to 


MOSSES   AND   LIVERWORTS  111 

note  that  in  its  earlier  stages  it  is  much  simpler  than 
the  mature  gametophyte,  but  closely  resembles  the  fully 
developed  thallus  of  certain  Hepaticse  whose  sporo- 
phyte  is  much  more  highly  developed  than  that  of 
Riccia. 

In  all  the  other  liverworts  the  sporophyte  shows  a 
certain  amount  of  vegetative  tissue,  only  a  portion  being 
devoted  to  the  formation  of  spores.  The  first  step  in 
this  separation  of  sporogenous  and  sterile  tissue  is  the 
division  of  the  fertilized  egg  into  two  cells  by  a  trans- 
verse wall,  the  upper  part  developing  into  the  spore- 
bearing  portion  or  "capsule,"  the  lower  giving  rise  to 
an  organ  of  absorption,  the  "foot"  (Fig.  28,  C,/),  and 
usually  an  intermediate  region,  which  forms  a  stalk  or 
pedicel  which  elongates  at  maturity,  and  causes  the 
sporophyte  to  rupture  the  archegonium-venter,  and  thus 
facilitates  the  scattering  of  the  spores.  In  most  of  thq 
Hepaticse  the  vegetative  tissue  develops  but  little  chloroA 
phyll,  and  the  growth  of  the  sporophyte  is  mainly  at  the  / 
expense  of  the  gametophyte,  from  which,  by  means  of  the; 
foot,  it  absorbs  nourishment  very  much  as  a  parasitic 
fungus  does  from  its  host.  In  all  of  the  Hepaticee,  except 
Riccia  and  one  or  two  closely  related  genera,  only  a 
part  of  the  sporogenous  tissue  or  archesporium  pro- 
duces perfect  spores.  The  others  either  remain  unde- 
veloped and  serve  to  nourish  the  growing  spores 
produced  from  the  other  cells,  or  more  commonly  they 
remain  undivided  and  form  peculiar  cells  known  as 
"elaters."  These  elongate  and  develop  upon  the  inner 
face  of  the  cell-wall  thickened  spiral  bands  which,  when 
fully  developed,  are  strongly  hygroscopic,  and  by  their 
movements,  induced  by  changes  in  moisture  after  the 


112  EVOLUTION  OF   PLANTS 

capsule  is  ripe,  help  to  distribute  the  spores  (Fig. 
28,  H). 

In  contrast  to  the  simple  sporophyte  of  the  lower 
liverworts,  there  is  found  in  one  group  a  sporophyte 
which  reaches  a  high  degree  of  complexity,  and  be- 
comes almost  independent  of  the  gametophyte.  This 
reaches  its  highest  expression  in  the  genus  Anthoceros 
(Fig.  28,  D,  Fig.  31,  C).  Here  the  gametophyte  is  very 
primitive  and  consists  of  a  simple  thallus  composed 
of  almost  perfectly  uniform  cells,  and  without  any  dif- 
ferentiation into  stem  and  leaves.  Indeed,  it  represents 
almost  the  lowest  type  of  the  gametophyte  among  the 
Hepaticse.  A  suggestion  of  an  origin  of  this  type  of 
thallus  from  the  Algae  is  seen  in  the  single  chloroplast 
in  each  cell,  much  like  that  in  Coleochaete.  The  sexual 
organs  of  Anthoceros,  while  on  the  whole  like  those  of 
the  other  liverworts,  are  peculiar  in  being  sunk  in  the 
thallus,  and  recall,  in  this  respect,  those  of  the  more 
primitive  ferns. 

It  is  the  sporophyte,  however,  which  is  of  the  greatest 
interest.  This  reaches  a  relatively  large  size  (Fig.  31, 
C,  SJP)  and  shows  a  considerable  degree  of  independent 
growth.  Between  the  large  foot  and  the  upper  portion 
is  a  zone  of  growing  tissue,  which  enables  the  sporophyte 
to  grow  in  length  as  long  as  the  gametophyte  remains 
active,  and  from  this  growing  zone  new  tissue  is  con- 
stantly added  to  the  base  of  the  sporophyte.  The  latter 
has  its  outer  parts  developed  into  a  perfect  assimilating 
tissue  with  several  layers  of  spongy  green  tissue  whose 
air-spaces  communicate  with  the  outside  atmosphere  by 
means  of  stomata  or  pores  in  the  epidermis,  precisely 
like  those  found  upon  the  leaves  of  the  higher  plants. 


MOSSES   AND   LIVERWORTS  113 

The  spores  arise  from  a  single  subepidermal  layer 
of  cells  (Fig.  28,  D),  which  later  becomes  deeper 
seated  through  the  further  division  of  the  superficial 
cells  (E).  Within  the  sporogenous  layer,  or  arche- 
sporium,  is  a  central  cylinder  of  sterile  cells  forming 
the  "columella,"  which  both  in  origin  and  position 
seems  to  represent  the  axial  vascular  bundle  or  strand 
of  conducting  cells  found  in  the  young  sporophyte  of 
the  ferns,  and  it  is  not  impossible  that  it  may  also  serve 
as  a  conducting  tissue,  thus  representing  a  primitive 
vascular  bundle  physiologically  as  well  as  structurally. 
Owing  to  the  absence  of  a  root  connecting  the  sporo- 
phyte with  the  earth,  it  remains  dependent  upon  the 
gametophyte  for  its  supply  of  water  and  also  for  cer- 
tain food  elements,  and  if  the  gametophyte  perishes,  the 
sporophyte  necessarily  soon  dies  as  the  supply  of  water 
is  cut  off.  Otherwise,  owing  to  the  perfect  assimilative 
system,  it  is  quite  independent,  and  if  a  root  were  pres- 
ent would  be  entirely  so. 

THE  TRUE  MOSSES  (Musci) 

The  second  class  of  the  Bryophytes,  while  greatly 
outnumbering  the  liverworts,  shows  very  much  less 
range  of  structure  and  is  evidently  a  much  more  spe- 
cialized group.  These  "True  Mosses,"  with  few  ex- 
ceptions, show  an  almost  stereotyped  plan  of  structure, 
the  differences  between  them  being  mostly  of  minor 
importance.  There  are  a  few,  however,  notably  the 
peat-mosses  (Sphagnacese),  which  show  affinities  with 
the  liverworts,  especially  with  Anthoceros. 

The  gametophyte  of  the  Musci  usually  exhibits  two 


114 


EVOLUTION  OF  PLANTS 


phases,  the  protonema  and  gametophore.  The  spore 
on  germination  produces  a  filamentous,  or  occasionally 
flat,  alga-like  growth,  the  protonema  (Fig.  29,  A,  B,  C, 

jt?r),  and  upon  this 
arise  special  buds  or 
branches  which  grow 
into  leafy  stems,  the 
gametophores  (Fig.  29, 
A,  B,  C,  &),  which  bear 
the  sexual  organs.  The 
leaves  of  the  gameto- 
phoric  branches  are 
commonly  arranged 
spirally,  and  the 
branches  seldom  are 
flattened  as  in  the 
foliose  Hepaticse. 
While  there  are  cer- 
tain superficial  resem- 
blances between  the 
leafy  stems  of  the 
mosses  and  foliose 
liverworts,  there  are 
differences  which  make 
it  extremely  improb- 
able that  the  former  have  been  derived  from  the  latter; 
The  two  forms  are  rather  to  be  considered  as  parallel 
developments.  In  the  Musci  the  structure  of  both  leaves 
and  stem  is  as  a  rule  much  more  complex  than  in  the 
Hepaticse,  and  there  is  usually  present  a  central  strand 
of  conducting  tissue,  quite  wanting  in  both  stem  and 
leaf  in  the  latter  group. 


'Pr 


FIG.  29  (Musci  or  True  Mosses).  — A,  the 
liverwort-like  protonema  of  a  peat- 
moss (Sphagnum)  with  the  leafy  shoot, 
k,  budding  out  from  it ;  B,  the  fila- 
mentous protonema,  pr,  of  a  common 
moss  (Funaria),  with  a  very  young 
leafy  bud,  k ;  C,  an  older  stage  of  the 
same  moss;  D,  the  full-grown  leafy 
gametophore,  g,  with  the  sporophyte, 
sp,  still  connected  with  it ;  ar,  the  re- 
mains of  the  archegonium  carried  up 
by  the  growth  of  the  sporophyte. 


MOSSES  AND   LIVERWORTS  115 

The  mosses  which  approach  most  nearly  to  the  He- 
paticse  are  undoubtedly  the  species  of  Sphagnum,  the 
common  peat-mosses.  In  these  the  protonema  arising 
from  the  germinating  spore  is  a  flat  thallus,  very  much 
like  a  simple  liverwort  in  appearance  (Fig.  29,  A). 
From  the  margin  of  this,  secondary  protonemal  branches 
arise  which  are  filamentous  and  closely  resembling  those 
of  the  higher  Musci. 

If,  as  seems  probable,  the  Musci  have  arisen  from 
the  Hepaticse,  Sphagnum  probably  represents  an  inter- 
mediate form,  and  the  flat,  liverwort-like  protonema 
must  be  considered  to  be  more  primitive  than  the  fila- 
mentous type  which  has  been  derived  secondarily  from 
it.  The  suppression  of  the  flat  thalloid  stage  is  prob- 
ably correlated  with  the  development  of  the  leafy 
gametophoric  branches,  which  become  more  and  more 
important. 

The  sporophyte  in  Sphagnum  is,  in  its  early  stages, 
remarkably  like  that  of  Anthoceros,  especially  in  the 
origin  of  the  archesporiurn.  Like  Anthoceros  the 
sporophyte  possesses  a  well-developed  assimilative  sys- 
tem of  green  tissue  with  numerous  stomata,  which  are 
not  always,  however,  functional. 

The  gametophyte  of  Sphagnum,  in  spite  of  its  large 
size,  shows  a  simpler  structure  than  that  of  the  typical 
mosses,  the  central  strand  of  tissue  being  absent  from 
the  stem,  and  the  leaves  being  destitute  of  a  midrib. 
There  are  a  few  forms  intermediate,  to  some  extent,  be- 
tween Sphagnum  and  the  typical  mosses  ;  but  a  very 
large  majority  of  the  Musci  belong  to  a  single  order, 
the  Bryinese.  While  these  show  great  diversity  in  their 
habits,  their  essential  structure  is  remarkably  uniform. 


116  EVOLUTION  OF  PLANTS 

They  occur  in  almost  all  situations  except  in  salt  water 
and  actual  deserts,  some  being  submersed  aquatics, 
others  growing  upon  the  ground,  or  upon  rocks  and 
trees,  and  indeed  in  any  situation  where  they  can  occa- 
sionally obtain  moisture.  Many  of  them  may  be  com- 
pletely dried  up  for  an  indefinite  period  without  losing 
their  vitality. 

In  the  growth  of  the  stem  and  leaves,  as  well  as  in  the 
structure  of  the  reproductive  organs,  the  gametophyte 
is  very  uniform.  Both  leaves  and  stem  show  a  definite 
apical  growth,  and  the  leaves  are,  with  few  exceptions, 
arranged  spirally  about  the  stem.  The  sexual  organs 
are  in  the  main  like  those  of  the  Hepaticse,  but  show  a 
definite  apical  growth  in  both  archegonium  and  anther- 
idium. 

The  sporophyte  is  highly  specialized  and  shows  a 
certain  degree  of  independence  in  the  development  of  a 
well-marked  assimilative  system  of  tissues,  as  in  Antho- 
ceros  and  Sphagnum.  It  differs  in  its  growth  from  that 
of  the  liverworts  in  the  presence  of  a  definite  single 
apical  cell,  to  whose  regular  divisions  the  early  growth 
of  the  embryo  is  due.  Later  this  apical  growth  is  re- 
placed by  a  basal  growth  much  as  in  Anthoceros. 

The  young  sporophyte  is  a  cylindrical  body  which 
later  develops  an  enlarged  upper  portion,  the  capsule  or 
theca,  borne  upon  a  long  stalk,  or  seta  (Fig.  29,  D,  sp). 
The  latter  is  usually  traversed  by  a  strand  of  conduct- 
ing tissue,  possibly  homologous  with  the  columella  in 
the  sporophyte  of  Anthoceros,  or  the  vascular  bundles 
in  the  young  fern-sporophyte. 

The  assimilative  tissue  in  the  sporophyte  of  the  higher 
Musci  is  very  perfect.  The  basal  part  of  the  capsule 


MOSSES  AND  LIVERWORTS 


117 


(Fig.  30,  A,  a)  is  composed  mainly  of  a  spongy  green 
tissue  which  is  also  present  in  the  upper  part  of  the 
capsule,  surrounding  the  large  air-spaces  between  the 
sporogenous  tissue 
and  the  outer  part 
of  the  capsule.  This 
green  tissue  recalls 
the  "  mesophyll  "  or 
spongy  green  tissue 
in  the  leaves  of  the 
higher  plants,  and 
like  the  mesophyll 
communicates  with 
the  outside  atmos- 
phere by  stomata.  In 
a  few  cases,  this  basal 
part  of  the  capsule 
(apophysis)  is  a  very 
much  enlarged  spe- 
cial organ,  comparable 
physiologically,  a  1  - 
though  hardly  struc- 
turally, with  the 
leaves  of  higher 
plants. 

The    formation    of 
spores  is  restricted  to 


CDC. 


FIG.  30  (Musci).  —  A,  a  longitudinal  section 
through  the  upper  part  of  the  sporophyte 
of  Funaria;  a,  the  apophysis  or  en- 
larged basal  part  of  the  capsule,  con- 
taining chlorophyll  and  with  stomata 
in  the  epidermis;  sp,  the  sporogenous 
tissue ;  o,  the  operculum,  or  lid,  which 
finally  falls  away  and  allows  the  escape 
of  the  spores  ;  r,  the  ring  of  cells  where 
the  lid  o  separates  from  the  urn,  or 
theca;  B,  cross-section  of  a  young  cap- 
sule, showing  the  position  of  the  sporo- 
genous cells;  C,  a  young  stoma  or 
breathing  pore  from  the  base  of  the 
capsule.  The  structure  of  the  stoma  is 
like  that  found  upon  the  leaves  of  the 
higher  plants. 


a  very  small   part  of 

the  sporophyte,  the  sporogenous  tissue  comprising  but  a 
single  layer  of  cells  forming  a  cylinder  in  the  middle 
region  of  the  capsule,  and  surrounding  the  central  col- 
umella  (B,  sp).  The  upper  part  of  the  capsule  usually 


118  EVOLUTION  OF   PLANTS 

becomes  detached  as  a  little  lid  (operculum),  and  the 
detachment  of  this  is  aided  by  the  formation  of  an 
elaborate  system  of  tooth-like  structures  (peristome) 
about  the  mouth  of  the  capsule.  These  teeth  are  ex- 
tremely hygroscopic,  and  by  their  movements  they  not 
only  help  to  throw  off  the  operculum,  but  also  to  empty 
the  capsule  and  disperse  the  spores,  which,  when  ripe, 
lie  loosely  in  the  capsule,  owing  to  the  drying  up  and 
withering  of  the  delicate  interior  tissues. 

SUMMARY 

While  there  is  little  question  that  the  Bryophytes 
have  arisen  from  forms  similar  to  certain  green  algse, 
it  must  be  admitted  that  so  far  as  existing  forms  are 
concerned  the  relationship  is  at  best  a  remote  one.  It 
is  true  a  direct  comparison  can  be  made  between  the 
sporophyte  in  Coleochsete,  for  example,  and  that  of 
Riccia,  and  the  change  from  the  motile  zoospores  of  the 
one  to  the  spores  of  the  other  can  be  explained  by  the 
abandonment  of  the  aquatic  habit  by  the  Bryophytes. 
The  gametophyte,  itself,  offers  no  serious  difficulties, 
retaining  in  Anthoceros,  for  instance,  apparently  the 
single  chloroplast  in  each  cell  found  in  so  many  algse, 
e.g.  Coleochsete,  and  the  structure  of  the  thallus  is 
hardly  more  complex  than  in  these ;  but  when  an 
attempt  is  made  to  compare  the  sexual  reproductive 
organs  it  must  be  admitted,  especially  as  regards  the 
archegonium,  that  the  difference  between  the  two 
groups  is  a  very  great  one.  The  nearest  approach  in 
this  respect  is  found  in  the  Characese,  which  otherwise 
differ  profoundly  from  the  Mosses,  and  so  far  as  our 


MOSSES  AND   LIVERWORTS  119 

knowledge  goes  at  present,  the  gulf  between  Algse  and 
Archegoniates  is  a  deep  one. 

The  dependence  of  all  Archegoniates  upon  water  for 
fertilization,  and  especially  the  presence  of  ciliated 
spermatozoids,  are  strong  arguments  for  the  derivation 
of  the  group  from  aquatic  ancestors,  but  at  present  this 
is  about  all  that  can  positively  be  asserted. 

Among  the  Archegoniates  themselves,  the  relation- 
ships are  much  more  obvious.  Undoubtedly  the  lowest 
forms  are  the  Hepaticse,  shown  both  by  comparison  with 
the  algae  and  with  the  other  Archegoniates,  and  probably 
these  are  to  be  considered  as  the  primitive  forms  from 
which  the  others  have  sprung. 

Among  the  Hepaticse,  the  lower  Jungermanniacese, 
such  as  Metzgeria  (Fig.  27,  C),  seem,  on  the  whole,  to  be 
the  simplest,  although  the  sporophyte  even  in  the  low- 
est ones  is  more  perfect  than  in  Riccia,  which  has  the 
lowest  type  of  sporophyte  found  among  the  Archegoni- 
ates. Assuming  that  the  lower  thallose  Jungermanni- 
acese  are  the  most  primitive  of  Hepatics,  we  have  seen 
that,  from  this  type,  several  others  have  been  developed. 
In  one  line  (Marchantiacese)  differentiation  has  resulted 
in  the  specialization  of  tissues,  the  plant  retaining  its 
primitive  thallose  form  (Fig.  27,  A,  B).  In  the  leafy 
liverworts,  the  tissues  have  remained  very  simple  and 
the  differentiation  has  been  purely  external,  resulting 
in  a  definite  axis  or  stem  bearing  three  rows  of  leaves 
(Fig.  27,  D,  E).  A  third  line  of  development  has  given 
rise  to  the  complex  leafy  gametophyte  of  the  true 
mosses. 

In  the  simpler  Hepaticse  the  sporophyte  is  small 
and  exclusively  devoted  to  spore-production,  e.g.  Riccia. 


120  EVOLUTION   OF  PLANTS 

In  the  higher  types  it  becomes  more  and  more  indepen- 
dent through  the  development  of  green  assimilative 
tissues.  This  reaches  its  highest  expression  in  Antho- 
ceros  and  the  Musci. 

The  latter  group  is  probably  the  most  modern  and 
specialized  one.  This  is  indicated  both  by  the  greater 
number  of  species  and  their  wider  distribution,  as  well 
as  by  a  much  more  stereotyped  structure.  These  have 
probably  arisen  from  liverworts  resembling  Anthoceros, 
and  it  is  not  likely  that  they  have  given  rise  to  any 
higher  forms,  but  represent  the  end  of  their  own  special 
line  of  development. 

In  the  evolution  of  the  sporophyte  there  has  been 
little  external  differentiation,  the  most  highly  special- 
ized forms  being  found  in  the  Musci,  where  the  sporo- 
phyte shows  a  foot  seta  and  capsule ;  but  there  are  no 
leaves  or  other  appendicular  organs,  although  tl)e  pecul- 
iar apophysis  found  in  a  few  mosses  perhaps  approaches 
this  condition. 

In  Anthoceros,  although  the  external  differentiation 
is  very  slight,  there  is  one  respect  in  which  it  stands 
alone,  i.e.  the  unlimited  growth  of  the  sporophyte. 
This,  in  connection  with  the  highly  developed  assimila- 
tive tissue,  makes  the  sporophyte  of  this  plant  the  near- 
est approach  to  the  entirely  independent  sporophyte  of 
the  ferns.  Were  the  foot  of  the  sporophyte  in  Antho- 
ceros prolonged  into  a  root  penetrating  the  earth,  it 
would  become  quite  independent  of  the  gametophyte, 
and  were  a  special  assimilate  organ  or  leaf  developed,  a 
condition  directly  comparable  to  the  sporophyte  of  the 
lower  Pteridophytes  or  ferns  would  result.  It  is  prob- 


MOSSES  AND  LIVERWORTS  121 

able  that  the  origin  of  the  latter  is  to  be  looked  for 
among  Hepaticse,  which  like  Anthoceros  had  a  very 
simple  gametophyte,  and  a  large,  nearly  self-supporting 
sporophyte  with  a  relatively  small  amount  of  sporoge- 
nous  tissue. 


CHAPTER   VII 

THE   FERNS    (PTEEIDOPHYTA} 

/ 

THE  Pteridophytes  or  Ferns,  using  the  latter  term  in 
its  widest  sense,  include  those  plants  sometimes  known 
as  the  Vascular  Cryptogams,  which  while  evidently  re- 
lated to  the  mosses  differ  from  them  in  the  very  much 
more  highly  developed  sporophyte,  which  here  becomes 
an  independent  plant.  Indeed,  it  is  the  sporophyte  or 
non-sexual  generation  of  the  ferns  which  is  the  plant 
as  it  is  ordinarily  understood,  the  gametophyte  being 
usually  small  and  inconspicuous  and  of  short  duration. 

It  will  be  remembered  that  in  considering  the  Bryo- 
phytes  great  differences  were  noted  in  the  relative 
development  of  gametophyte  and  sporophyte;  that 
while  in  Riccia,  for  example,  the  sporophyte  is  nothing 
more  than  a  capsule  filled  with  spores,  in  Anthoceros 
the  spore-formation  is  subordinated  to  a  considerable 
extent,  and  there  is  developed  a  well-marked  assimila- 
tive issue,  consisting  of  green  cells  with  large  intercellu- 
lar spaces,  and  stomata  communicating  with  the  outside 
as  in  the  vascular  plants.  Moreover,  this  sporophyte  is 
not  limited  in  its  growth,  but  continues  to  elongate  as 
long  as  the  gametophyte  remains  alive.  Owing  to  the 
absence  of  a  root,  however,  the  sporophyte  still  remains 
dependent  upon  the  gametophyte  for  water,  and  to 
some  extent  for  food  also;  but  the  well-developed 

122 


THE   FERNS 


123 


green  tissue  enables  it  to  utilize  the  carbon  dioxide  of 
the  atmosphere. 

It  is  not  a  long  step  from  such  a  sporophyte  as  that 
of  Anthoceros  to  that  of  the  lower  Pteridophytes.  In 
the  latter,  owing  to  the 
early  development  of  a 
root  in  the  sporophyte, 
the  latter  soon  becomes 
quite  independent  of  the 
gametophyte,  which  is 
generally  short  lived, 
although  occasionally  it 
reaches  a  considerable 
size  and  may  live  for 
several  years,  especially 
where  the  sporophyte 
fails  to  develop  (Fig. 
31,  A). 

The  sporophyte  in 
even  the  lowest  Pterido- 
phytes exhibits  a  com- 
plexity far  exceeding 
that  of  the  highest 
moss.  This  is  especially 

the  case  in  regard  to  the  external  differentiation.  While 
in  all  Bryophytes  there  is  very  little  development  of 
special  external  members  in  the  sporophyte,  in  ferns 
there  are  very  early  developed  several  characteristic 
external  organs,  viz.,  stem,  leaf,  and  root.  The  foot,  or 
absorbent  organ  of  the  embryo,  is  much  like  the  corre- 
sponding organ  in  the  moss-embryo. 

Corresponding  to  this  development  of  external  mem- 


FIG.  31.  — A,  gametophyte  of  a  fern 
(Marattia) ,  showing  a  forking  of  the 
growing  point,  and  the  development 
of  secondary  buds,  k  ;  B,  gameto- 
phyte of  the  same  fern,  with  the 
young  sporophyte,  sp,  attached ;  C, 
a  liverwort,  Anthoceros,  with  several 
sporophytes,  sp,  attached  to  the 
gametophyte,  g.  The  sporophyte  is 
capable  of  long-continued  growth, 
but  does  not  develop  a  root,  and 
hence  never  becomes  entirely  inde- 
pendent. 


124  EVOLUTION   OF   PLANTS 

bers,  the  tissues  of  the  sporophyte  show  a  much  greater 
degree  of  complexity  than  is  found  in  any  of  the  plants 
below  the  ferns.  This  is  especially  seen  in  the  develop- 
ment of  the  so-called  "  vascular  bundles,"  which  are  met 
with  for  the  first  time  in  their  fully  developed  con- 
dition in  the  sporophyte  of  the  ferns.  These  tissues 
are,  however,  hinted  at  in  the  sporophytes  of  some  of 
the  mosses.  Thus  the  central  strand  of  tissue  in  the 
seta  of  the  moss-sporogonium,  and  the  columella  in 
Anthoceros,  both  in  origin  and  appearance,  suggest  the 
young  vascular  bundles  in  the  organs  of  the  young 
fern-embryo,  and  may  probably  be  fairly  considered 
as  the  homologues  of  these. 

It  is  in  the  ferns,  however,  that  we  first  encounter 
the  peculiar  tracheary  tissue  characteristic  of  the  woody 
portions  of  the  bundles  in  the  vascular  plants.  This 
tracheary  tissue  is  made  up  of  empty  cells  with  woody 
walls,  and  is  a  very  important  element  in  the  conduc- 
tion of  water  in  the  vascular  plants.  These  empty  cells 
are  known  as  tracheids,  but  occasionally  in  the  ferns 
there  are  encountered  true  vessels,  or  rows  of  tracheids 
whose  partition  walls  have  been  absorbed.  In  the  ordi- 
nary ferns  the  woody  tissue  or  "  xylem  "  is  surrounded 
by  a  mass  of  "phloem"  or  "bast,"  containing  as  its 
most  characteristic  element  the  sieve-tubes,  similar  in 
appearance  to  the  tracheary  tissue  of  the  xylem,  but 
without  lignified  walls  and  containing  living  proto- 
plasm. The  vascular  bundles  form  a  complicated  system 
of  strands  in  the  stem  of  the  sporophyte,  and  with  these 
are  connected  the  bundles  traversing  the  roots  and 
leaves. 

A  well-marked  epidermal  tissue   is  always   present, 


THE   FERNS  125 

especially  well  developed  upon  the  leaves,  where  it  is 
furnished  with  stomata  communicating  with  the  green 
tissue  of  the  leaf,  and  also  is  often  provided  with  hairs 
and  scales  of  characteristic  form.  The  remaining  tissue, 
usually  known  as  the  "ground-tissue,"  shows  a  much 
greater  diversity  of  structure  than  is  met  with  in  any  of 
the  lower  plants,  and  closely  approaches  in  this  respect 
the  higher  flowering  plants. 

While  in  the  mosses  the  existence  of  the  sporophyte 
usually  ends  with  the  dispersal  of  the  spores,  in  the 
ferns  spore-formation  is  subordinated  to  the  vegetative 
existence  of  the  sporophyte.  The  spores  themselves, 
instead  of  arising  from  a  large,  continuous  archesporium, 
are  here  restricted  to  certain  definite  structures  of  the 
sporophyte  called  sporangia  (Figs.  34,  35).  A  faint 
indication  of  this  segregation  of  the  sporogenous  tissue 
is  seen  in  the  Anthocerotacese,  among  the  liverworts, 
where  there  is  an  imperfect  separation  of  small  sporo- 
genous areas  by  means  of  sterile  tissue  between  them. 

In  the  ferns,  as  a  rule,  the  development  of  spores 
usually  takes  place  only  after  the  sporophyte  has 
reached  an  advanced  stage  of  development,  and  this 
is  often  not  accomplished  for  many  years  in  some  of 
the  large  ferns,  although  in  a  few  cases  the  sporophyte 
lives  but  a  single  season. 

A  study  of  the  development  of  an  individual  case 
illustrates  very  clearly  the  homologies  which  exist  be- 
tween ferns  and  the  lower  mosses.  It  is  well  known 
to  botanists  that  the  germinating  fern-spore  does  not 
at  once  produce  the  leafy  sporophyte,  but  there  is  first 
formed  a  much  simpler  plant,  the  gametophyte  (Fig.  32). 
On  first  germinating,  the  unicellular  spore  usually  pro- 


126 


EVOLUTION   OF  PLANTS 


duces  a  slender  filament  or  cell-row,  much  like  the  sim- 
pler green  algae.  This  condition  soon  gives  place  to  a 
delicate  flat  thallus,  closely  resembling  some  of  the 

simpler  liver- 
worts. At  this 
stage  growth  is 
effected  by  a 
single  apical  cell 
(Fig.  32,  B,  x) 
precisely  as  in 
such  simple  liv- 
erworts as  An- 
eura.  The  degree 
of  development 
of  this  thalloid 
gametophyte 
varies  much  in 

FIG.  32.  —  A,  the  germinating  spore  of  the  ostrich  different      ferilS, 

fern  (Onoclea  struthiopteris)  ,  showing  the  rup-  Vyrif  if  rna,V  reach 
tured  spore-coat,  sp,  and  the  first  rhizoid,  r;  B,  * 

a  somewhat  older  plant  (gametophyte)  with  a  a  length  of  S6V- 
single   apical  cell,    x;    C,    female  gametophyte 

seen  from  below,  showing  the  archegonia,  ar;  eral  Centimetres, 

D,  young  sporophyte,  sp,  still  attached  to  the  -,  -,  . 

gametophyte,  g\  the  sporophyte  has  developed  Drancmng 

leaves  and  roots,   r,   so  that  it  is  quite   inde-  4-QT,0:,T«iTr 

pendent  of  the  gametophyte.  tensively, 

living    for    sev- 

eral years,  especially  when  the  archegonia  remain  unfer- 
tilized (Fig.  31,  A).  The  largest  of  these  "prothallia  " 
occur  in  certain  tropical  ferns,  especially  species  of 
filmy-ferns  (Hymenophyllacese)  and  Vittaria.  In  the 
latter  genus  they  sometimes  have  numerous  branches, 
radiating  from  a  common  centre  and  forming  circular 
disks  ten  centimetres  or  more  in  diameter,  and  closely 
resemble  a  large  liverwort.  These  large  gametophytes 


6X- 


THE   FERNS  127 

are  usually  sterile,  and  seem  to  be  the  result  of  exces- 
sive vegetative  activity.  They  not  infrequently  multi- 
ply by  means  of  special  buds,  or  gemmae,  by  which  the 
number  of  the  gametophytes  may  be  rapidly  increased 
exactly  as  in  the  liverworts. 

In  some  species  of  Trichomanes  (Fig.  35,  E),  a  genus 
of  the  filmy-ferns,  the  gametophyte  may  have  the  form 
of  an  extensively  branched  filament,  closely  resembling 
an  alga ;  and  it  has  been  suggested  that  this  may  be  the 
primitive  type  of  the  gametophyte.  However,  as  many 
closely  allied  species  produce  the  usual  flat  thallus,  and 
all  of  the  forms,  when  exposed  to  excessive  moisture, 
show  a  tendency  to  assume  a  filamentous  stage,  it 
is  quite  as  likely  that  this  is  an  adaptation  to  a  moist 
environment,  rather  than  being  the  primary  condition. 

Another  group  of  ferns,  the  so-called  Eusporangiatse, 
which  includes  the  adder-tongue  (Ophioglossum)  (Fig. 
34,  A)  and  its  allies,  as  well  as  certain  interesting  trop- 
ical forms,  the  Marattiacese  (Figs.  31,  34),  show  a  long- 
lived  gametophyte  of  a  somewhat  different  type.  In  all 
of  these,  so  far  as  they  are  known,  the  gametophyte  is 
massive  and  quite  different  from  the  thin,  delicate 
thallus  of  the  filmy-ferns  and  Vittaria,  but  like  these  the 
gametophyte  may  live  for  a  long  time,  often  for  several 
years,  and  not  infrequently  remains  alive  long  after  the 
young  sporophyte  is  quite  independent.  The  gameto- 
phyte in  the  Marattiaceae,  especially  (Fig.  32,  A,  B), 
is  extraordinarily  like  a  simple  thallose  liverwort,  both 
as  regards  the  thallus  itself  and  the  sexual  organs  devel- 
oped upon  it.  In  the  adder-tongues  the  gametophyte, 
so  far  as  at  present  known,  is  subterranean  and  quite 
destitute  of  chlorophyll ;  but  whether  this  is  originally 


•UNIVERSITY 


128  EVOLUTION   OF   PLANTS 

the  case  remains  to  be  seen,  as  the  earliest  stages  are 
very  imperfectly  known. 

The  Horse-tails  (Equisetinese)  (Fig.  36)  and  the 
Club-mosses  (Lycopodineye)  (Figs.  37,  38),  while  differ- 
ing in  some  minor  details,  agree  closely  in  the  main  with 
the  eusporangiate  ferns  in  the  characters  of  the  gameto- 
phyte. 

Upon  this  thalloid  gametophyte  are  borne  the  repro- 
ductive organs,  antheridia  and  archegonia,  structurally 
very  much  like  those  of  the  Bryophytes,  especially  the 
liverworts,  which  with  little  question  are  the  nearest 
relatives  of  Pteridophytes  among  the  lower  plants.  The 
resemblances  are  especially  marked  in  the  Anthocero- 
taceae,  which  are  also  the  nearest  approach  to  the  ferns 
in  the  structure  of  the  sporophyte. 

Within  the  antheridium  are  produced  motile  sperma- 
tozoids,  which,  in  the  true  ferns,  have  many  cilia  (Fig. 
33,  C)  instead  of  the  two  possessed  by  the  moss-sperma- 
tozoid,  and  these  require  the  presence  of  water  in  order 
that  they  may  reach  the  egg-cell  in  the  open  arche- 
gonium ;  and  water  is  also  necessary,  as  in  the  Bryo- 
phytes, for  the  opening  of  the  ripe  reproductive  organs. 

We  have  already  indicated  in  a  preceding  chapter 
that  the  motile  spermatozoids  of  the  algse  are  to  be  con- 
sidered as  modifications  of  originally  non-sexual  zoo- 
spores,  which  in  turn  are  a  reversion  to  the  originally 
free-swimming  ancestral  type  from  which  all  the  green 
plants  originated.  The  recurrence  of  these  ciliated  re- 
productive cells  in  the  Pteridophytes  is  a  strong  argu- 
ment in  favor  of  considering  these  plants  as  being  also 
derived  from  originally  aquatic  ancestors.  Fertilization 
is  effected  in  these  as  in  the  mosses,  and  the  gameto- 


THE   FERNS 


129 


phyte  may  be  described  as  amphibious,  inasmuch  as  it 
must  become  aquatic,  so  to  speak,  in  order  that  fertili- 
zation may  be  effected. 

The  spermatozoid,  attracted  by  the  substance  ejected 
from  the  open  archegonium,  swims  to  it  and  makes  its 
way  through  the 
canal  in  the  neck 
to  the  central 
cavity,  where  it 
quickly  pene- 
trates the  egg- 
cell  and  slowly 
fuses  with  its 
nucleus,  after 
undergoing  a  se- 
ries of  changes. 
As  a  result  of 
fertilization  the 
egg  begins  to 
grow,  having 
in  the  mean 
time  secreted  a 
wall  about  itself, 
and  thus  forms 
what  may  be 
called  a  spore, 

comparable  to  the  resting-spore  of  such  green  algae  as 
(Edogonium,  or  to  the  fertilized  egg-cell  in  the  moss 
archegonium.  Like  the  latter  it  germinates  at  once 
instead  of  passing  through  a  long  dormant  period,  as  in 
most  green  algse. 

The  early  divisions  in  the  young  embryo,  developed 


FIG.  33. —  A,  the  open  archegonium  of  the  ostrich- 
fern,  showing  the  egg-cell,  o,  within  the  venter; 
B,  the  antheridium  of  the  same  species ;  C,  a 
free  spermatozoid,  showing  the  numerous  cilia ; 
D,  the  fertilized  archegonium  containing  the 
young  embryo  sporophyte ;  E,  the  archegonium 
of  a  liverwort,  Riccia,  with  the  young  sporophyte, 
showing  the  close  resemblance  between  the  ferns 
and  mosses  in  regard  to  the  young  sporophyte  ; 
F,  longitudinal  section  of  an  older  embryo  of 
the  ostrich  fern,  showing  the  division  into  stem, 
st ;  leaf,  L;  root,  R;  and  foot,  F. 


130  EVOLUTION  OF   PLANTS 

from  the  egg,  agree  exactly  with  those  in  the  liverwort- 
embryo,  and  the  great  similarity  in  the  structure  of  the 
young  sporophyte  in  Bryophytes  and  Pteridophytes 
(Fig.  33,  D,  E)  is  one  of  the  strongest  evidences  of  the 
intimate  relationship  of  the  two  great  divisions  of  the 
ArchegoniatSB.  The  young  embryo  consists  at  first  of 
four  nearly  equal  cells,  arranged  like  the  quadrants  of  a 
sphere,  and  in  the  lower  ferns  the  young  sporophyte 
retains  this  globular  or  oval  form  for  a  considerable 
time,  and  closely  resembles  the  corresponding  stages  in 
certain  low  liverworts,  e.g.  Riccia.  In  the  common 
ferns,  however,  there  very  early  appears  a  marked  devi- 
ation from  the  type  found  in  the  mosses.  This  is  the 
indication  of  external  members,  absent  in  the  embryo  of 
the  latter.  Usually  each  of  the  four  original  quadrants 
of  the  young  embryo  becomes  the  starting-point  for  a 
special  organ,  and  soon  these  are  evident  as  the  rudi- 
ments of  the  primary  leaf  or  cotyledon,  the  stem  or 
axis  of  the  young  sporophyte,  the  primary  root,  and  the 
foot  (Fig.  33,  F).  Each  of  these  organs  in  the  more 
specialized  ferns  shows  a  definite  apical  cell,  and  this 
apical  growth  in  each  of  the  members  soon  causes  the 
young  sporophyte  to  assume  the  character  of  an  inde- 
pendent plant,  the  young  fern,  in  short.  The  root 
elongates  rapidly  and  soon  fastens  the  young  sporo- 
phyte to  the  earth,  and  as  soon  as  the  primary  leaf  is 
expanded,  the  little  fern  is  quite  independent  of  the 
gametophyte  with  which  it  is  still  connected  by  means 
of  the  foot,  through  which  it  is  nourished  until  its  own 
primary  members  are  fully  developed  (Fig.  32,  D). 

In  the  more  generalized  and  lower  ferns,  the  sporo- 
phyte retains  much  longer  its  undifferentiated  character, 


THE   FERNS  131 

and  is  dependent  upon  the  gametophy te  for  a  long  period 
—  indeed  in  some  of  these  the  gametophy  te  remains 
alive  for  months,  or  even  years,  after  the  sporophyte 
has  become  quite  capable  of  self-support. 

It  is  the  development  in  the  sporophyte  of  these  ex- 
ternal members  —  stem,  leaf,  and  root  —  which  at  once 
distinguishes  the  fern  from  the  moss,  and  it  is  the  pres- 
ence of  these  which  enables  the  sporophyte  to  become 
independent  of  the  gametophyte,  which  soon  perishes. 
It  must  be  remembered,  however,  that  the  young  sporo- 
phyte in  the  ferns  is  also  dependent  for  a  longer  or 
shorter  period  upon  the  gametophyte,  just  as  is  the  case 
permanently  in  the  mosses,  and  the  cases  known  among 
the  former,  where  the  existence  of  the  gametophyte 
does  not  necessarily  end  when  the  sporophyte  has  be- 
come independent,  recalls  at  once  the  normal  condition 
of  things  among  the  Bryophytes. 

Of  the  original  quadrants  into  which  the  fern-embryo 
divides,  one,  as  we  have  seen,  becomes  the  apex  of  the 
future  stem,  and  this  cell  may  retain  its  identity,  persist- 
ing as  the  apical  cell  of  the  axis  of  the  plant.  Thus  in 
the  gigantic  tree-ferns,  the  single  initial  cell  at  the  apex 
of  the  stem  is  the  direct  descendant  of  one  of  the  four 
primary  cells  into  which  the  embryo  was  first  divided. 
The  growth  of  all  of  the  other  original  members  of  the 
embryo  is  limited,  the  cotyledon  and  primary  root  very 
soon  dying  and  giving  place  to  others. 

The  size  which  the  sporophyte  finally  reaches  varies 
extremely.  Thus  in  some  of  the  tiny  filmy-ferns  (Fig. 
35,  C)  the  delicate  stem  is  hardly  thicker  than  a  hair, 
and  the  fully  developed  leaves  less  than  a  centimetre 
in  length ;  on  the  other  hand,  some  of  the  giant  tree- 


132  EVOLUTION  OF  PLANTS 

ferns  may  have  an  erect  stem  ten  or  fifteen  metres  in 
height,  with  leaves  five  or  six  metres  long.  These  gi- 
gantic sporophytes  offer  a  strong  contrast  to  the  insig- 
nificant sporophyte  of  the  mosses,  and  corresponding  to 
this  is  the  late  appearance  of  the  sporogenous  tissue, 
which  may  not  be  formed  until  after  many  years.  This 
extreme  subordination  of  the  sporogenous  tissue  is  a 
wide  departure  from  the  condition  existing  in  such  low 
liverworts  as  Riccia,  where  practically  the  whole  sporo- 
phyte is  composed  of  sporogenous  cells. 

In  all  of  the  Pteridophytes  the  sporogenous  tissue  is 
restricted  to  certain  definite  areas,  these  being  confined 
to  more  or  less  distinct  organs,  sporangia.  The  latter 
are  possibly  foreshadowed  by  the  imperfect  segregation 
of  the  sporogenous  tissue  in  the  Anthocerotacese,  the 
highest  of  the  liverworts.  Among  the  ferns,  the  forms 
which  approach  nearest  the  condition  existing  Ain  the 
Anthocerotaceae  are  the  species  of  Ophioglossum  or 
adder-tongues,  where  the  limits  of  the  sporangia  are 
scarcely  indicated  at  all  upon  the  surface  (Fig.  34, 
A,  B,  C).  In  these  ferns  the  sporogenous  tissue  occurs 
in  masses  of  considerable  size,  but  is  not  clearly  sepa- 
rated from  the  surrounding  tissue.  The  archesporial 
cells  are  separated  from  the  epidermis  of  the  leaf 
(sporophyll)  by  several  layers  of  cells,  and  the  spores 
finally  escape  through  a  cleft  which  opens  at  the  sur- 
face of  the  sporophyll.  The  archesporium  is  at  first 
of  sub-epidermal  origin,  as  in  Anthoceros,  the  latter 
being  in  this  particular  more  like  the  ferns  than  like 
the  typical  mosses,  where  the  sporogenous  cells  are 
originally  derived  from  the  central  part  of  the  sporo- 
phyte. Even  in  Anthoceros,  however,  the  separate 


THE    FERNS 


133 


groups  of  sporogenous  cells  are  much  less  definite  than 
in  Ophioglossum,  and  do  not  have  a  separate  opening 
for  each ;  still  it  is  quite  conceivable  that  the  simple 
sporangia  of  Ophioglos- 
sum may  have  originated 
from  structures  not  unlike 
the  groups  of  sporogenous 
cells  found  in  the  Antho- 
cerotacese. 

In  the  ferns,  as  in  the 
mosses,  each  sporogenous 
cell  gives  rise  to  four 
spores,  which  develop  in 
an  absolutely  similar  way, 
and  offer  another  striking 
proof  of  the  close  rela- 
tionships of  the  two 

groups.  FlG    34    (Eusporangiate    Ferns).  — A, 

If  we  admit  that  Ophio-         sporophyte  of  an  adder-tongue  fern 

glossum  shows  the  most 
primitive  type  of  spo- 
rangium among  the  ferns, 
we  may  say  that  passing 
from  this  type  to  that 
found  in  the  most  special- 
ized ferns,  the  so-called 
"  Leptosporangiatse,"  we 
encounter  an  almost  perfect  series  of  intermediate  forms 
leading  up  to  the  exceedingly  specialized  sporangium 
of  the  latter,  and  this  series  may  be  assumed  to  rep- 
resent the  evolution  of  the  sporangium  of  the  lepto- 
sporangiate  ferns  from  the  simpler  type  found  in 


(Ophioglossum  vulgatum) ;  sp,  the 
sporangial  spike ;  B,  longitudinal 
section  of  the  sporangial  spike 
of  another  species  (0.  pendulum'), 
showing  the  cavities  containing  the 
spores ;  C,  cross-section  of  the  spo- 
rangial spike  of  0.  pendulum  ,•  D, 
separate  sporangia  of  Botrychium ; 
E,  leaflet  of  Marattia,  showing  the 
synangia,  or  coherent  sporangia;  F, 
a  single  synangium  cut  to  show  the 
separate  sporangial  cavities ;  G, 
Angiopteris,  one  of  the  Marattiacese 
with  nearly  separate  sporangia,  sp. 


134  EVOLUTION   OF   PLANTS 

Ophioglossum.  This  specialization  consists  in  a  more 
definite  limitation  of  the  sporangium,  and  the  restric- 
tion of  the  primary  sporogenous  -tissue  or  archesporium 
to  a  single  cell  (Fig.  35,  E,  F).  The  genus  Botry- 
chium,  which  is  obviously  related  to  Ophioglossum, 
has,  within  its  limits,  species  which  illustrate  the 
change  from  a  large  indefinite  sessile  sporangium  much 
like  that  of  Ophioglossum,  e.g.  B.  simplex,  to  the 
much  smaller  obviously  stalked  sporangium  found  in 
such  large  species  as  B.  Virginianum  (Fig.  34,  D). 
This  evolution  of  the  sporangium  is  accompanied  by  a 
growing  complexity  in  the  divisions  of  the  leaf,  as  well 
as  correlated  modifications  of  the  tissues  of  the  sporo- 
phyte,  which  approximate  the  structures  of  the  typical 
Leptosporangiatee.  Another  group  of  ferns,  intermedi- 
ate in  some  respects  between  the  lower  (Eusporangiatse) 
and  the  leptosporangiate  ferns,  are  the  Osmundaceae, 
including  the  royal  fern  Osmunda  regalis,  and  in  east- 
ern America  the  common  cinnamon-fern,  0.  cinna- 
momea.  In  these  the  sporangia  and  the  tissues, 
especially  the  vascular  bundles,  show  undoubted  re- 
semblances to  the  Eusporangiatse,  although,  on  the 
whole,  they  are  nearer  the  leptosporangiate  type. 

In  the  latter  the  sporangia  can  be  traced  back  to  a 
single  epidermal  cell,  the  early  divisions  of  which  are 
extremely  regular,  and  result  in  the  formation  of  a 
single  central  archesporial  cell  surrounded  by  a  single 
layer  of  outer  cells.  The  so-called  "tapetal  cells" 
(Fig.  35,  F,  t)  are  cut  off  from  the  archesporium,  but 
are  later  broken  down,  so  that  at  maturity  the  wall  of 
the  sporangium  consists  of  a  single  layer  of  cells.  A 
constant  character  of  these  ferns  is  the  formation  of 


THE  FERNS 


135 


st 


the  "annulus,"  a  row   of   cells  with   thickened  walls 
whose  contraction  plays  an  important  part  in  the  open- 
ing of  the  ripe  sporangium  and  the  discharge  of  the 
spores  (Fig.  35,  G, 
H,r). 

On  comparing 
the  Eusporangiatse 
and  Leptosporan- 
giatee,  one  is  at 
once  struck  by  the 
great  disparity  in 
the  numbers  of  the 
two  groups.  Prob- 
ably all  living 
species  of  Euspo- 
rangiatse,  including 
the  peculiar  genus 
Isoetes,  whose  posi- 

sorus  covered  with  "the  "kidney-shaped  In- 


tion  here  is  by  no 
means  certain, 
scarcely  exceed  one 
hundred,  while  the 
Leptosporangiates, 
the  typical  ferns, 
number  probably 
at  least  3500  to 


FIG.  35  (Leptosporangiate  Ferns).  — A,  leaflet 
of  a  shield-fern  (Aspidium),  showing  the 
sori,  or  sporangial  groups,  s  ;  B,  a  single 
sorus  covered  with  the  kidney-shaped  in- 
dusium,  in;  C,  a  filmy-fern  (Trichomanes) 
with  the  sorus  surrounded  by  a  trumpet- 
shaped  indusium ;  D,  longitudinal  section 
of  the  sorus,  showing  the  sporangia  borne 
upon  the  elongated  columella;  E,  F,  young 
sporangia  of  Poli/podium  falcatum  seen  in 
section ;  the  sporogenous  cell  is  shaded ; 
t,  t,  the  tapetal  cells  which  later  are  broken 
down;  G,  a  ripe  sporangium  of  the  same 
species  showing  the  ring  or  annulus,  r,  and 
the  stomium,  st,  where  the  opening  occurs ; 
H,  sporangium  of  a  climbing  fern  (Lygo- 
dium)  with  terminal  annulus,  r. 


4000    species.     In 

spite  of  this  extraordinary  difference  in  numbers  of 
species,  the  former  group  shows  much  greater  range 
of  structure,  so  much  so  that  it  is  necessary  to  make 
two  and  perhaps  three  orders  to  include  them,  and  the 
relationships  of  these  are  very  doubtful.  The  Lepto- 


136  EVOLUTION   OF  PLANTS 

sporangiates,  in  spite  of  their  numbers,  might  all  be 
included  in  a  single  order,  Filices,  were  it  not  for 
a  small  number  of  the  heterosporous  forms,  i.e.,  those 
having  two  kinds  of  spores,  which  are,  however,  evi- 
dently related  to  the  Filices.  A  study  of  the  different 
orders  of  the  Eusporangiatse  indicates  that  we  have  to 
do  with  remnants  of  once  much  larger  groups,  of  which 
most  of  the  members  have  become  extinct.  The  much 
greater  homogeneity  of  the  Leptosporangiatte,  as  well 
as  their  numbers,  indicate  on  the  other  hand  a  special- 
ized and  presumably  more  modern  type  of  vegetation, 
and  this  is  borne  out  by  a  study  of  their  distribution. 

None  of  the  Eusporangiatse  ever  occur  in  any  great 
numbers  together,  although  some  of  them  are  cosmo- 
politan. One  order,  the  Marattiacese,  are  strictly  tropi- 
cal plants,  and  usually  occur  as  isolated  individuals  or 
in  small  groups.  Among  the  Leptosporangiates,  on 
the  other  hand,  the  plants  are  often  gregarious,  and 
form  conspicuous  features  of  the  vegetation.  The  com- 
mon brake,  Pteris  aquilina,  and  in  the  tropics,  species  of 
Gleichenia,  form  tangled  thickets  and  cover  extensive 
tracts  almost  to  the  exclusion  of  other  vegetation.  If 
we  analyze  the  fern-flora  of  those  tropical  regions 
where  ferns  form  an  important  feature  of  the  vege- 
tation, the  disproportion  in  numbers  between  the  eu- 
sporangiate  and  leptosporangiate  species  is  even  greater 
than  in  temperate  regions.  Thus  in  Jamaica,  which  is 
exceptionally  rich  in  ferns,  out  of  about  five  hundred 
species  described  from  the  island,  less  than  a  dozen  are 
eusporangiate,  and  of  these  none  are  common  enough 
to  make  any  impression  upon  the  general  character  of 
the  vegetation,  although  an  occasional  gigantic  Marattia 


THE   FERNS  137 

attracts  the  attention  of  the  botanist.  The  Leptosporan- 
giates,  on  the  other  hand,  occur  everywhere  in  the  most 
astonishing  profusion  and  variety,  often  constituting 
the  most  conspicuous  feature  of  the  flora  of  certain 
districts,  especially  in  the  higher  mountains. 

The  conclusion  is  irresistible  that  in  the  Leptospo- 
rangiatse  we  have  to  do  with  a  comparatively  modern 
type  of  plants,  eminently  adapted  to  existing  conditions 
and  competing  successfully  with  the  highly  specialized 
flowering  plants.  The  small  number,  both  of  species 
and  individuals,  among  the  Eusporangiates  points  to  the 
opposite  condition  in  their  case.  They  show  every  evi- 
dence of  plants  that  are  being  worsted  in  the  struggle 
for  existence  by  their  more  specialized  competitors. 

A  study  of  the  anatomy  of  the  sporophyte  as  well  as 
the  gametophyte  confirms  this  view.  We  find  that  the 
gametophyte  in  the  Eusporangiates  approaches  that  of 
the  liverworts  much  more  closely  both  in  its  structure 
and  long  duration,  and  the  reproductive  organs  are 
much  more  like  the  liverwort  type  than  are  those  of 
the  leptosporangiate  ferns.  The  embryo  also  remains 
much  longer  connected  with  the  gametophyte,  and  the 
differentiation  of  its  members  does  not  take  place  until 
a  later  period.  Finally,  the  sporophyte  has  simpler 
tissues,  and  the  sporangium  is  of  a  less  specialized 
type,  approximating  the  conditions  found  in  the  highest 
liverworts.  In  short,  the  theory  of  the  Eusporangiatae 
being  primitive  and  presumably  an  older  type  than  the 
Leptosporangiatse  is  borne  out  by  every  detail  of  their 
structure. 

That  the  Leptosporangiates  have  been  derived  from 
the  Eusporangiates  is  indicated  by  the  number  of 


138  EVOLUTION   OF   PLANTS 

transitional  forms,  like  Osmunda,  which  connect  per- 
fectly the  two  groups.  We  should  naturally  expect  that 
the  most  specialized  forms,  i.e.  those  which  have  diverged 
most  widely  from  the  primitive  stock,  would  be  the  pre- 
vailing forms  at  the  present  time,  and  such  really  is  the 
case.  The  Polypodiacese,  which  include  all  the  com- 
moner ferns  and  are  with  little  question  the  most 
specialized  of  the  ferns,  far  outnumber  all  the  other 
families  combined,  and  are  preeminently  the  modern 
type. 

It  is  interesting  to  note  that  the  conclusions  reached 
by  a  study  of  comparative  morphology  are  confirmed  by 
the  geological  record.  The  oldest  ferns  known  are  be- 
yond question  Eusporangiates,  all  of  the  ferns  found 
in  the  Carboniferous  and  pre-Carboniferous  rocks  prob- 
ably being  of  this  character,  while  undoubted  Lepto- 
sporangiates  first  appear  in  the  Mesozoic  formations, 
from  which  time  they  appear  to  have  increased  in  num- 
ber and  variety,  gradually  replacing  the  eusporangiate 
ferns  of  the  earlier  formations.  There  is  no  evidence 
that  the  Leptosporangiates  have  ever  been  any  more 
abundant  than  at  the  present  time,  and  they  are  prob- 
ably to  be  considered  as  a  distinctly  modern  type. 


CHAPTER   VIII 

PTERIDOPHYTA  —  Concluded 

BESIDE  the  true  ferns  there  are  two  other  classes  of 
existing  Pteridophytes,  —  the  Equisetineae  (horsetails, 
scouring-rushes)  and  the  Lycopodineae  or  club-mosses. 

The  former,  while  showing  certain  points  of  resem- 
blance to  the  ferns,  still  differ  so  widely  from  them  that 
they  are  properly  included  in  a  separate  class.  All  the 
known  living  forms  belong  to  a  single  genus,  Equise- 
tum,  which  comprises  about  twenty-five  species,  mostly 
belonging  to  the  northern  hemisphere,  and  especially 
well  represented  in  the  United  States.  The  peculiar 
sporophyte  (Fig.  36,  A),  with  its  jointed,  grooved  stems, 
and  sporiferous  cones,  is  familiar  to  every  botanist. 

The  gametophyte  is  less  generally  known  and  shows 
many  points  of  resemblance  to  that  of  the  ferns,  espe- 
cially the  Eusporangiatae.  The  green  spores  germinate 
promptly  if  sown  as  soon  as  they  ripen,  but  soon  lose 
their  power  of  germination.  After  about  a  month  the 
male  gametophyte  is  mature,  the  female  plant  requir- 
ing a  somewhat  longer  time.  In  its  earlier  stages  the 
gametophyte  is  much  like  that  of  the  common  ferns, 
but  is  more  irregular  in  shape,  developing  more  or  less 
definite  lobes,  which  are  especially  conspicuous  in  the 
female  plant.  The  growth  of  the  latter  is  a  good  deal 
like  that  of  the  gametophyte  of  Marattia  or  Osmunda, 

.139 


140 


EVOLUTION   OF   PLANTS 


except  for  the  conspicuous  lobes  referred  to  above. 
The  reproductive  organs  are  very  much  like  those  of 
the  eusporangiate  ferns,  and  the  spermatozoids,  which 
are  large  and  multiciliate,  closely  resemble  those  of 
Osmunda. 


FIG.  36  (Equisetinefe).  —  A,  upper  part  of  a  sporiferons  shoot  of  a  horse- 
tail (Equisetum  pratense),  showing  the  division  into  nodes  and  inter- 
nodes,  the  rudimentary  sheath-leaves,  sh,  and  the  strobilus  or  cone  of 
sporophylls,  c:  B,  a  cross-section  of  an  internode  of  E.  maximum, 
showing  the  arrangement  of  the  vascular  bundles,  v,  and  the  air-spaces, 
or  lacuna?,  I :  C,  longitudinal  section  of  the  apex  of  a  young  shoot  of  E. 
maximum,  showing  the  single  large  apical  cell,  x  :  D,  a  single  sporophyll 
of  the  same  species  with  the  sac-shaped  sporangia,  sp ;  E,  median  sec- 
tion of  the  sporophyll ;  F,  a  ripe  spore,  with  the  elaters,  el. 


The  sporophyte,  however,  shows  many  points  of  dif- 
ference which  are  early  manifest.  Thus,  in  the  embryo, 
it  is  the  stem-quadrant  which  grows  most  actively, 
while  the  development  of  the  leaves  is  subordinated  to 
it,  as  it  is  throughout  the  life  of  the  sporophyte.  In- 
stead of  the  short  stem  and  large  leaves  of  the  ferns, 
the  stem  in  Equisetum  is  very  much  elongated,  while  the 
leaves  are  reduced  to  the  toothed  sheaths  which  sur- 


PTERIDOPHYTA  141 

round  the  nodes  or  joints  of  the  stem.  These  reduced 
leaves  are  practically  useless  as  assimilative  organs,  and 
their  office  is  assumed  by  the  internodes  of  the  stem 
and  branches,  where  the  green  tissue  is  largely  devel- 
oped, and  connected  with  the  outside  by  numerous 
stomata  in  the  epidermis.  The  leaves  serve  as  protec- 
tive organs  only,  forming  a  thick  covering  over  the 
apex  of  the  young  shoot,  and  also  covering  the  buds 
from  which  spring  the  lateral  branches. 

In  studying  the  development  of  the  tissues  of  the 
sporophyte,  one  is  struck  by  the  almost  mathematical 
regularity  in  the  divisions  of  the  cells  at  the  stem-apex, 
as  well  as  in  the  roots.  The  shoot  in  all  species  termi- 
nates in  a  single  apical  cell  (Fig.  36,  C),  having  the 
form  of  an  inverted  three-sided  pyramid  from  whose 
lateral  faces  segments  are  cut  off  in  regular  succession, 
and  the  tissues  of  the  mature  stem  bear  a  definite 
relation  to  the  early  divisions  in  these  segments.  A 
similar  regularity  exists  in  the  early  divisions  of  the 
cells  at  the  apex  of  the  root.  The  stem  is  traversed  by 
a  regular  system  of  lacunae,  or  air-passages  (Fig.  36, 
B,  £),  and  the  vascular  bundles  are  arranged  in  a  circle, 
recalling  the  arrangement  in  the  stem  of  the  typical 
Dicotyledons.  In  the  arrangement  of  the  woody  tissue 
and  bast,  they  recall  the  flowering  plants  rather  than 
the  ferns,  although  among  the  latter  the  Ophioglossacese 
show  a  somewhat  similar  type  of  vascular  bundle,  —  the 
"collateral"  form,  —  and  also  other  structural  resem- 
blances. 

The  sporangia  of  Equisetum  occur  upon  peculiar  um- 
brella-shaped sporophylls  which  are  arranged  in  whorls 
about  the  apex  of  certain  shoots,  and  crowded  together  so 


EVOLUTION  OF  PLANTS 

as  to  form  a  cone  or  strobilus.  In  their  development,  the 
sporangia  are  much  like  those  of  the  eusporangiate  ferns, 
but  in  their  method  of  opening -they  are  more  like  the 
sporangia  (anthers)  of  the  flowering  plants.  The  early 
stages  in  the  development  of  the  spores  follow  the  reg- 
ular type  found  in  all  Archegoniates,  but  the  ripe  spore 
is  very  peculiar,  being  provided  with  curious  appendages 
(elaters),  formed  by  a  splitting  of  the  outer  membrane 
(Fig.  36,  F). 

The  existing  species  of  Equisetum  differ  a  good  deal 
in  size,  varying  from  small  forms  not  more  than  ten  to 
twenty  centimetres  in  height,  to  the  giant  of  the  genus, 
E.  giganteum  of  tropical  America,  which  may  reach  a 
height  of  ten  metres,  with  a  stem  diameter  of  two  or 
three  centimetres.  In  spite  of  these  differences  in  size 
they  all  agree  closely  in  the  structure  of  the  sporo- 
phyte. 

While  all  the  living  members  of  the  class  can  be 
placed  in  a  single  genus,  it  is  different  with  the  numer- 
ous fossil  forms  which  are  known.  Especially  during 
the  Carboniferous  epoch  was  there  a  rich  development  of 
this  peculiar  group  of  plants,  which  formed  a  conspicu- 
ous feature  in  the  vegetation,  where  they  were  repre- 
sented by  numerous  genera  and  species.  The  modern 
genus  Equisetum  probably  extends  back  to  the  coal- 
measures,  where  it  was  associated  with  numerous  extinct 
types  which  reached  a  far  greater  size  and  complexity. 
The  largest  of  the  fossil  Equisetinese  were  the  species  of 
Calamites,  which  attained  tree-like  dimensions  and  whose 
remains  show  evidences  of  a  secondary  thickening  of 
the  vascular  bundles  of  the  stem,  like  that  in  the  trunks 
of  existing  trees.  It  is  interesting  that  a  trace  of  this 


PTERIDOPHYTA 


143 


peculiarity  has  been   recently  detected  in  one  of   the 
living  species  of  Equisetum. 

Associated  with  some  of  the  fossil  forms  there  are 
found  cones,  which  evidently 
belong  with  them,  and  resemble 
those  of  the  existing  Equisetum. 
In  a  few  instances  they  have 
been  preserved  so  perfectly  that 
the  inner  structure  can  be  accu- 
rately made  out,  and  it  is  evi- 
dent that  the  tissues  and  spo- 
rangia of  these  plants  closely 
resembled  those  of  Equisetum, 
although  most  of  them  exhibit 
a  degree  of  specialization  not 
found  in  any  of  their  living 
relatives.  The  Equisetinese  rap- 
idly diminish  in  importance  in 
the  later  geological  epochs,  until, 
as  we  have  seen,  but  a  single 
genus  has  survived  to  the  pres- 
ent time,  and  this  is  one  of  the 
less  specialized  types. 


FIG.  37  (Lycopodine?e).  — A, 
part  of  a  plant  of  a  club- 
moss  (Lycopodium  clava- 
tum)  with  two  sporangia! 
spikes,  sp  ;  B,  a  sporophyll 


LYCOPODINE^E 


from  the  spike  of  L.  den- 
droideum,  bearing  a  single 
large  sporangium,  sp ;  C, 
cross-section  of  the  stem  ; 
vb,  the  central  vascular 
cylinder. 


The   third   class    of    Pterido- 

phytes,  the  Club-mosses,  is  intermediate  in  point  of  num- 
bers between  the  two  already  considered.  There  are 
three  well-marked  orders,  of  which  the  first,  Lycopodi- 
acese,  includes  the  common  club-mosses  belonging  to 
the  genus  Lycopodium.  The  second  order,  Selaginel- 


144  EVOLUTION  OF   PLANTS 

lacese,  or  smaller  club-mosses,  is  closely  related  to  the 
Lycopodiacese,  and  includes  a  single  genus  Selaginella, 
with  several  hundred  species,  mostly  tropical  (these  are 
common  in  greenhouses,  where  they  are  usually  mis- 
named "Lycopodium  ").  The  third  order,  Psilotacese, 
includes  two  peculiar  tropical  genera,  Psilotum  and 
Tmesipteris,  evidently  closely  related  genera,  but  doubt- 
fully associated  with  the  other  Lycopods,  and  possibly 
more  nearly  allied  to  certain  extinct  Pteridophytes. 

The  gametophyte  is  at  present  known  only  in  Lyco- 
podium and  Selaginella,  and  until  its  character  in  the 
other  genera  is  known,  it  will  be  impossible  to  assign 
them  their  proper  place  in  the  system.  In  Lycopodium 
the  gametophyte  varies  greatly  in  different  species,  in 
some  being  a  green  lobed  thallus  somewhat  like  the 
gametophyte  in  Equisetum,  while  in  others  it  is  desti- 
tute of  chlorophyll,  at  least  in  its  older  stages,  and  is 
apparently  truly  saprophytic  in  its  habits.  The  earliest 
stages  of  these  colorless  gametophytes  are  not  known, 
and  it  is  possible  that  they  may  at  first  possess  chloro- 
phyll. The  sexual  organs  are  much  like  those  of  Equi- 
setum and  the  eusporangiate  ferns,  but  the  spermatozoids 
have  only  two  cilia,  as  in  the  Bryophytes. 

The  embryo  in  the  club-mosses  differs  from  that  of  the 
other  Pteridophytes  in  being  derived  from  one  only  of 
the  two  cells  resulting  from  the  first  transverse  division 
of  the  egg-cell.  The  other  cell  forms  a  structure  known 
as  the  suspensor  (Fig.  38,  G,  «MS),  which  is  much  like  the 
similar  organ  found  in  the  embryo  of  most  flowering 
plants.  The  embryo  in  Lycopodium  remains  for  a  long 
time  dependent  upon  the  gametophyte,  and  may  develop 
several  leaves  before  the  first  root  is  formed. 


PTERIDOPHYTA 


145 


In  Selaginella  (Fig.  38),  while  the  embryo  closely 
resembles  that  of  Lycopodium,  the  gametophyte  is  very 
different.  The  sporophyte  produces  two  sorts  of  spores, 
large  and  small.  The  former,  the  macrospores,  produce 
a  rudimentary  gametophyte,  which  bears  only  archegonia 
(Fig.  38,  E).  The  gametophyte  projects  from  the  spore 
but  little,  and  until  its  later  stages  is  contained  entirely 
within  the  macrospore.  In  germination  there  are  first 


FIG.  38  (LycopodinefB) . — A,  a  branch  of  one  of  the  smaller  cluh-mosses 
(Selaginella)  with  two  sporangial  spikes,  .sp ;  B,  longitudinal  section  of 
spike  showing  a  single  macrosporangium,  ma,  and  several  microsporan- 
gia,  mi ;  C,  germinated  microspore  containing  the  rudimentary  male 
gajnetophyte ;  v,  the  single  vegetative  cell;  an,  the  antheridium;  D,  a 
spermatozoid  (after  Belajeff) ;  E,  germinating  maorospore  with  the 
female  gametophyte  protruding;  ar,  archegonia;  F,  a  single  arche- 
gonium  ;  G,  a  young  embryo,  em,  attached  to  the  suspensor,  sus,  whose 
base  remains  within  the  archegonium ;  H,  young  sporophyte,  still  at- 
tached to  the  gametophyte  within  the  macrospore;  cot,  cotyledons; 
r,  root. 

produced  within  the  spore  numerous  free  nuclei,  be- 
tween which,  later,  cell-walls  arise,  forming  a  continuous 
tissue  much  as  in  the  "  embryo-sac "  of  the  flowering 
plants.  The  formation  of  the  gametophyte  begins  in 
Selaginella  before  the  spores  are  set  free  from  the 
sporangium.  The  small  spores  or  microspores  produce 
an  even  more  rudimentary  gametophyte  (C),  which  is 


146  EVOLUTION  OF  PLANTS 

reduced  to  a  single  vegetative  cell,  and  a  single  anthe- 
ridium  in  which  are  developed  biciliate  spermatozoids 
like  those  of  Lycopodium  (D)* 

In  both  Lycopodium  and  Selaginella,  the  stem  of  the 
sporophyte  is  long  and  extensively  branched,  while  the 
leaves  are  small  and  moss-like.  The  tissues,  especially 
the  vascular  bundles,  are  not  unlike  those  of  the  ferns. 
Sometimes  the  stem  and  root  grow  from  a  single  apical 
cell,  sometimes  a  group  of  initial  cells  is  present,  but 
even  when  there  is  a  single  apical  cell,  it  never  shows  the 
almost  mathematical  regularity  in  its  divisions  found  in 
the  leptosporangiate  ferns  or  in  Equisetum. 

The  sporangia  in  both  Lycopodium  and  Selaginella 
are  borne  singly,  either  upon  the  inner  face  of  the 
leaves,  or  upon  the  axis  just  above  a  leaf.  They  are 
kidney-shaped  capsules,  which  open  by  a  longitudinal 
cleft  (Fig.  87,  B).  The  sporophylls  are  usually 
crowded  together  into  a  cone  or  strobilus,  somewhat 
as  in  Equisetum.  In  Lycopodium  all  the  sporangia  are 
alike,  but  in  Selaginella  the  oldest  one  (or  ones),  at  the 
base  of  the  cone,  matures  but  four  spores  (macrospores), 
which  are  very  much  larger  than  the  numerous  micro- 
spores  produced  in  the  upper  sporangia  (Fig.  38,  B). 
The  development  of  the  two  kinds  of  spores  is  the  same 
up  to  the  point  where  each  mother-cell  divides  into  the 
four  spores.  In  the  microsporangia  all  the  spores  de- 
velop, but  in  the  macrosporangium  only  one  tetrad  comes 
to  maturity,  the  others  serving  simply  as  food  for  the 
developing  macrospores.  These  begin  to  germinate 
within  the  sporangium,  and  besfdes  using  up  the  other 
spore-tetrads  as  food,  are  nourished  from  the  sporophyte 
through  the  cells  of  the  sporangium- wall,  which  re- 


PTERIDOPHYTA  147 

main  alive  and  active  up  to  the  time  the  spores  are 
ripe. 

A  third  genus,  Phylloglossum,  allied  to  Lycopodium, 
includes  a  single  species  from  Australia,  and  is  appar- 
ently a  very  primitive  type,  as  it  resembles  closely  the 
embryonic  condition  of  some  species  of  Lycopodium. 
Unfortunately,  all  attempts  to  germinate  the  spores 
have  failed,  and  the  gametophyte  is  entirely  unknown. 

The  order  Psilotacese,  which  is  commonly  associated 
with  the  club-mosses,  includes  two  tropical  genera, 
Psilotum  and  Tmesipteris.  They  are  usually  epi- 
phytes, i.e.  grow  upon  the  trunks  and  branches  of 
trees,  and  Tmesipteris  shows  some  evidences  of  being 
partially  parasitic.  The  sporangia  are  large  and  all 
alike,  but  as  yet  nothing  is  known  of  the  nature  of  the 
gametophyte  produced  from  them,  so  that  it  is  impossi- 
ble to  compare  it  with  that  of  the  other  Pteridophytes, 
and  at  present  the  systematic  position  of  these  curious 
plants  must  be  regarded  as  doubtful. 

Like  the  Equisetinese,  the  club-mosses  were  once 
much  more  abundant  than  at  present,  and  many  of 
them  far  exceeded  in  size  and  complexity  any  of  the 
existing  species.  Members  of  this  class  probably  ex- 
isted as  far  back  as  the  upper  Devonian,  and  in  the  Car- 
boniferous rocks  they  form  one  of  the  most  conspicuous 
features  of  the  fossil  flora.  The  most  striking  forms 
are  the  species  of  Sigillaria  and  Lepidodendron,  which 
reached  tree-like  dimensions  and  showed  a  secondary 
thickening  of  the  stems  like  that  of  the  living  conifer- 
ous trees.  Many  of  these  fossil  Lycopods  are  preserved 
in  an  extraordinarily  perfect  manner,  so  that  the  histo- 
logical  details  are  perfectly  recognizable  and  can  readily 


148  EVOLUTION   OF  PLANTS 

be  compared  with  those  of  existing  species.  Occasion- 
ally even  the  spore-bearing  parts  have  been  well  pre- 
served, and  it  is  evident  that  Lepidodendron  and  its 
allies  were  structurally  much  like  the  living  genera 
Lycopodium  and  Selaginella.  Especially  does  Lepido- 
dendron resemble  the  latter  in  the  character  of  the 
spores,  which  are  of  two  kinds,  macrospores  and  micro- 
spores.  The  genus  Lycopodium  seems  to  be  very  old, 
fossils  apparently  very  close  to  the  living  species  occur- 
ring in  the  older  rocks.  These  simpler  forms  have  held 
their  own  in  the  struggle  for  existence,  while  the  more 
highly  specialized  ones  seem  to  have  been  crowded  out 
by  the  still  more  specialized  seed  plants,  some  of  which 
may  be  their  direct  descendants. 

HETEROSPORY 

In  all  of  the  principal  groups  of  Pteridophytes,  in 
passing  from  the  simpler  to  the  more  specialized  forms, 
a  striking  phenomenon  manifests  itself,  i.e.  "hetero- 
spory,"  or  the  development  of  two  sorts  of  spores,  produc- 
ing respectively  male  and  female  gametophytes.  In  the 
lower  members  of  each  series,  the  "  homosporous  "  forms, 
the  spores  are  all  alike,  and  on  germination  produce 
a  thallus  of  considerable  size  showing  more  or  less 
resemblance  to  that  of  the  lower  liverworts,*  and  in 
extreme  cases  living  for  several  years.  Upon  this 
thallus  the  reproductive  organs  are  borne,  antheridia 
and  archegonia  usually  growing  upon  the  same  plant, 
but  sometimes  upon  separate  ones.  Where  the  gameto- 
phyte  is  unisexual,  as  in  Equisetum  and  some  ferns, 
the  male  plants  are  smaller,  in  extreme  cases  being 


PTERIDOPHYTA  149 

reduced  to  a  few  vegetative  cells  and  a  single  anthe- 
ridium.  In  all  of  these  homosporous  types,  however, 
there  is  nothing  in  the  appearance  of  the  spore  to 
indicate  whether  the  resulting  gametophyte  is  to  be 
male  or  female,  and  indeed  this  is  sometimes,  to  a 
certain  extent  at  least,  a  matter  of  nutrition. 

In  each  of  the  principal  groups  of  Pteridophytes,  how- 
ever, we  find  at  least  one  genus  which  develops  two 
very  distinct  forms  of  spores,  i.e.  is  heterosporous.  In 
all  but  the  Equisetinese  there  are  existing  examples  of 
heterosporous  genera,  but  in  the  latter  class  the  single 
living  genus  is  homosporous,  although  some  of  its  fossil 
relatives  are  known  to  have  been  heterosporous. 

Among  the  eusporangiate  ferns  it  is  an  open  question 
whether  there  are  any  undoubted  cases  of  heterospory, 
although  it  is  probable  that  the  peculiar  genus  Isoetes 
(Fig.  39,  A),  where  heterospory  is  very  pronounced,  is 
related,  although  remotely,  to  the  homosporous  Euspo- 
rangiatae.  It  certainly  seems  to  be  nearer  to  the  ferns 
than  to  the  club-mosses  with  which  it  is  usually  asso- 
ciated. 

In  Isoetes  the  sporangia  (Fig.  39,  B),  which  are  very 
large,  and  borne  singly  at  the  bases  of  the  closely  crowded 
rush-like  leaves,  are  alike  in  structure  and  external  form, 
but  there  is  an  enormous  difference  in  the  size  of  the  ma- 
crospores  and  microspores.  The  male  plant  produced 
from  the  microspore  (Fig.  39,  C)  is  exceedingly  rudimen- 
tary, consisting  of  a  single  minute  vegetative  cell  and  an 
antheridium  which  produces  but  four  spermatozoids,  and 
is  the  most  reduced  known  among  the  Pteridophytes, 
and  approximates  nearest  the  condition  found  in  the 
flowering  plants.  The  macrospore  is  very  large  and 


150 


EVOLUTION   OF  PLANTS 


filled  with  accumulated  food  substances  which  serve  to 
supply  the  developing  female  gametophyte  with  food, 
as  the  latter  does  not  contain  chlorophyll.  The  gam- 
etophyte, as  in  Selaginella,  is  almost  entirely  included 
within  the  large  macrospore,  and  the  formation  of  the 


sp 


FIG.  39  (Heterosporous  Ferns).  —  A,  sporophyte  of  Isoetes  echinospom;  B, 
a  single  leaf  showing  the  enlarged  base  bearing  a  single  macrosporan- 
gium,  ma ;  the  microsporangia  are  much  the  same;  C,  a  germinated 
microspore  with  the  contained  gametophyte  reduced  to  a  single  vegeta- 
tive cell,  v,  and  an  antheridium  with  four  coiled  spermatozoids ;  D, 
Marsilia  vestita,  a  heterosporous  form  allied  to  the  leptosporangiate 
ferns;  sp,  the  "  sporocarp"  or  modified  leaf-segment  within  which  are 
borne  the  sporangia ;  E,  section  of  the  upper  part  of  the  macrospore 
and  female  gametophyte,  here  reduced  to  a  single  archegonium,  or; 
the  body  of  the  macrospore,  sp,  remains  undivided;  F,  spermatozoid  of 
Marsilia ;  x,  the  remains  of  the  central  part  of  the  sperm-cell. 


cells  is  preceded  by  a  repeated  division  of  the  nuclei  as 
in  the  formation  of  the  gametophyte  or  "endosperm"  of 
the  flowering  plants.  Germination,  however,  does  not 
begin  until  the  spores  have  been  set  free.  The  arche- 


PTERIDOPHYTA  151 

gonia  in  Isoetes  are  very  much  like  those  of  the  euspo- 
rangiate  ferns,  and  the  spermatozoids  are  multiciliate 
like  those  of  the  typical  ferns,  and  it  is  largely  for  these 
reasons  that  the  writer  is  inclined  to  consider  Isoetes  as 
related  to  the  ferns  rather  than  to  the  club-mosses. 

Among  the  Leptosporangiatse  heterospory  has  devel- 
oped quite  independently  in  at  least  two  places.  The 
two  families,  Marsiliacese  and  Salviniacese,  usually  asso- 
ciated under  the  name  of  Hydropterides  or  water-ferns, 
are  obviously  not  closely  related,  and  they  show  evidence 
of  having  been  derived  independently  from  two  widely 
separated  families  of  homosporous  ferns.  The  Salvini- 
acese  show  certain  resemblances  to  the  filmy  ferns,  while 
the  Marsiliacese  are  more  like  the  Polypodiacese.  Both 
families  agree  in  having  the  macrospores  reduced  to  a 
single  one  in  each  macrosporangium,  through  the 
abortion  not  only  of  the  other  spore-tetrads,  but  also 
of  the  three  sister-spores  of  the  macrospore.  The  latter 
becomes  very  large,  and  its  outer  membranes  much 
modified  (Fig.  39,  E). 

In  the  Salviniacese,  especially  Salvinia,  the  female 
gametophyte  is  much  larger  than  in  the  Marsiliacese, 
or  indeed  than  in  any  other  heterosporous  Pteridophyte. 
It  has  abundant  chlorophyll  and  does  not  differ  very 
essentially  from  the  green  gametophyte  of  the  homo- 
sporous ferns.  The  male  plant,  too,  is  less  reduced  than 
in  other  heterosporous  forms.  In  the  Marsiliacese,  the 
female  gametophyte  is  reduced  to  little  more  than  a 
single  archegonium  (Fig.  39,  E),  and  the  male  plant  to 
a  single  antheridium  with  one  or  two  rudimentary 
vegetative  cells. 

In  the  genus  Marsilia  the  development  by  the  gameto- 


152  EVOLUTION   OF   PLANTS 

phyte  is  exceedingly  rapid,  in  marked  contrast  to  the 
long-lived  gametophyte  of  the  homosporous  ferns.  The 
ungerminated  dried  spores  of  Marsilia  vestita  (Fig.  39, 
D),  for  example,  a  common  species  of  the  western 
United  States,  on  being  placed  in  water  will  complete 
their  whole  development  within  less  than  twenty-four 
hours,  the  sexual  organs  being  matured  and  fertilization 
effected  within  that  time. 

In  the  Equisetineee,  heterospory,  as  already  noted, 
is  known  only  in  a  few  fossil  forms,  and  in  these  there 
is  much  less  difference  in  the  size  of  the  two  sorts  of 
spores  than  is  the  case  in  the  heterosporous  ferns. 

The  club-mosses,  as  we  have  seen,  show  very  marked 
heterospory  in  the  genus  Selaginella,  which  includes  the 
majority  of  the  existing  species,  mostly  tropical  in  their 
distribution.  In  Selaginella,  as  in  Isoetes,  the  formation 
of  the  female  gametophyte  is  preceded  by  a  „ repeated 
division  of  the  nucleus  of  the  macrospores,  and  closely 
resembles  the  endosperm  formation  of  the  flowering 
plants.  The  male  gametophyte  is  reduced  to  a  single 
vegetative  cell  as  in  Isoetes,  but  the  number  of  sperm- 
cells  is  much  greater,  and  the  spermatozoids  are  biciliate 
as  in  Lycopodium  or  the  mosses,  and  not  multiciliate 
like  those  of  the  other  Pteridophytes. 

In  Selaginella  the  germination  of  the  spores  begins 
while  they  are  still  included  in  the  sporangium,  whose 
wall-cells  remain  active,  the  inner  layer  of  cells  acting  as 
nourishing  cells  for  the  developing  spores  with  the  con- 
tained gametophyte.  The  latter  derives  its  sustenance, 
not  from  reserve  matter  within  the  spore,  but  directly  from 
the  sporophyte.  In  this  respect  Selaginella  approaches 
the  condition  found  in  the  flowering  plants,  where  the 


PTERIDOPHYTA  153 

macrospore   remains    permanently   within   the   sporan- 
gium. 

SUMMARY 

In  reviewing  the  Pteridophytes  or  Ferns,  we  have 
seen  that  the  three  existing  classes  are  sharply  sepa- 
rated from  each  other  by  the  characters  of  the  sporo- 
phyte.  In  the  ferns  proper  the  leaves  are  greatly  de- 
veloped, while  the  stem  is  often  short  and  inconspicuous. 
In  the  other  two  classes,  the  horsetails  and  club-mosses, 
it  is  the  stem  which  is  especially  developed,  while  the 
leaves  are  small  and  sometimes  quite  functionless  as 
organs  of  assimilation,  as  seen  in  Psilotum  or  Equise- 
tum. 

In  the  lower  or  homosporous  members  of  all  the 
series,  the  gametophyte  is  comparatively  long-lived,  and 
there  is  a  good  deal  of  similarity  of  structure  in  all  of 
them,  especially  in  regard  to  the  sexual  organs.  Both 
the  gametophyte  itself  and  the  sexual  organs  show 
marked  resemblances  to  certain  liverworts,  especially 
the  Anthocerotaceae.  These  are  so  great  as  to  war- 
rant the  assumption  of  an  origin  of  the  Pteridophytes 
from  liverwort-like  ancestors  which  must  have  resem- 
bled in  many  respects  the  Anthocerotacese. 

The  resemblances  between  ferns  and  Equisetum  in 
the  structure  of  the  reproductive  organs,  and  especially 
the  spermatozoids,  are  very  marked,  and  suggest  a 
possible  common,  but  very  remote,  origin  for  the  two. 
The  small  biciliate  spermatozoids  of  the  Lycopods,  on 
the  other  hand,  seem  to  indicate  a  more  direct  origin 
of  these  from  forms  like  existing  liverworts ;  but  as 
yet  no  Bryophytes  are  known  which  possess  the  large 


154  EVOLUTION  OF  PLANTS 

multiciliate  spermatozoids  of  the  ferns.  There  is  no 
very  satisfactory  evidence  of  the  origin  of  any  of  the 
existing  classes  from  either  of  the  others,  although  there 
are  certain  characters  which  the  lower  members  of  all 
the  series  have  in  common.  It  is  probable  that  all  have 
originated  from  either  the  same  or  closely  related  ances- 
tral forms,  but  the  three  classes  as  they  now  exist  may 
be  considered  as  coordinate. 

The  geological  evidence  shows  conclusively  that  the 
club-mosses  and  horsetails  are  to  be  considered  as 
remnants  of  groups  once  much  more  important  than  at 
present,  which  probably  reached  their  maximum  devel- 
opment during  the  Carboniferous  era. 

With  the  ferns  the  matter  seems  different.  Of  the 
two  main  divisions,  the  Eusporangiatse,  i.e.  the  Maratti- 
acese  and  Ophioglossacese,  show  strong  evidence  of  being 
primitive  forms.  This  is  indicated  not  only  by  the 
large  long-lived  gametophyte  and  the  form  of  the  re- 
productive organs,  but  also  by  the  simplicity  of  the  tis- 
sues of  the  sporophyte,  especially  the  undifferentiated 
sporangia,  which  show  an  approach  to  the  condition 
found  in  certain  liverworts.  The  evidence  of  compar- 
ative anatomy  is  confirmed  by  the  geological  record, 
which  shows  conclusively  that  the  oldest  fossil  ferns 
were  undoubtedly  of  the  eusporangiate  type.  The 
Marattiacese,  especially,  were  very  much  better  repre- 
sented than  at  present. 

From  the  primitive  eusporangiate  stock,  which,  as 
might  naturally  be  expected,  shows  certain  affinities 
with  the  lower  members  of  the  Lycopodinese  and  Equi- 
setinese,  the  more  specialized  and  modern  Leptosporan- 
giatse  have  arisen,  and  at  present  they  form  the  prevail- 


PTERIDOPHYTA  155 

ing  type  of  Pteridophytes,  which  has  largely  crowded  out 
the  more  primitive  Eusporangiates.  Certain  genera, 
like  Osmunda,  are  probably  intermediate  in  character 
between  the  two.  The  Leptosporangiatse  have  diverged 
further  and  further  away  from  the  parent  stock,  reach- 
ing their  highest  expression  in  the  heterosporous  forms 
like  Marsilia  and  Salvinia.  It  is  doubtful  whether  the 
latter  have  given  rise  to  any  higher  types. 

It  is  possible  that  another  important  group  of  plants, 
the  Angiosperms  or  highest  of  the  flowering  plants,  has 
arisen  from  the  Eusporangiatee.  There  are  numerous 
striking  resemblances  in  the  structures  of  the  two 
groups,  and  it  is  possible  that  the  peculiar  genus  Isoetes 
may  represent  a  transitional  condition.  The  relation  of 
the  latter  to  the  ferns  is  by  no  means  admitted  by  all 
botanists,  but  on  the  whole  it  seems  to  be  more  nearly 
related  to  these  than  to  the  Lycopods.  If  this  is  true, 
it  is  not  impossible  that  from  some  similar  forms  the 
lower  Monocotyledons  have  arisen. 

Another  group  of  flowering  plants,  admittedly  the 
lowest  of  all,  shows  almost  certain  affinity  with  the 
eusporangiate  ferns.  These  are  the  Cycads,  whose 
recently  discovered  spermatozoids  break  down  the  last 
barrier  between  ferns  and  flowering  plants. 

The  Equisetinese,  so  far  as  we  can  judge,  never  de- 
veloped beyond  the  large  heterosporous  forms  found 
fossil,  but  the  Lycopods,  through  forms  like  Selaginella, 
and  much  larger  but  similar  fossil  types  like  Lepidoden- 
dron,  may  perhaps  have  been  the  ancestors  of  a  part  at 
least  of  the  Gymnosperms,  or  lowest  group  of  seed-bearing 
plants.  The  similarity  of  the  tissues  of  the  sporophyte, 
and  especially  the  remarkable  resemblances  in  the  gam- 


156  EVOLUTION   OF   PLANTS 

etophyte  and  embryo  of  Selaginella  and  the  Conifers, 
are  very  noticeable,  and  in  connection  with  Conifer-like 
Lepidoclendrons  and  other  arborescent  ancient  types, 
suggest  a  direct  origin  for  the  Conifers  from  such 
ancestral  forms. 

We  see,  then,  that,  starting  from  a  common  form,  or 
at  least  from  similar  ancestral  forms,  probably  allied  to 
existing  liverworts,  the  three  existing  classes  of  Pterido- 
phytes  have  developed  along  parallel  lines.  In  all  cases 
there  has  been  a  reduction  of  the  gametophyte  among  the 
higher  members  of  each  series,  with  a  corresponding 
perfecting  of  the  sporophyte.  This  has  resulted  finally 
in  heterospory,  which  in  at  least  two  cases  —  i.e.  eu- 
sporangiate  ferns  and  Lycopods  —  has  resulted  in  the 
production  of  seed-bearing  plants.  In  the  one  case  the 
result  was  the  Angiosperms  and  perhaps  the  Cycads ; 
in  the  other  the  Conifers.  From  the  Eusporangiatse 
were  also  developed,  as  a  second  branch,  the  modern 
group  of  leptosporangiate  ferns. 

The  accompanying  diagram  will  show  the  relations  of 
these  groups. 


PTERIDOPHYTA 


Angiosperuise 


Heterosporese 


Leptosporangiatse 
Isosporese 


Heterosporese 


Cycadacese 


Heterosporese 


Isosporepe 


Isosporese 


Coniferse  (?) 


Heterosporese 


Isosporese 


Eusporangiatae 


Horsetails  Ferns  Club-mosses 

(Equisetinese)    (Filicinese)     (Lycopodinese) 


Liverworts 
(Hepaticae) 

Diagram  to  illustrate  the  relationships  between  the  three  existing 
classes  of  Pteridophytes  and  the  Spermatophytes. 


CHAPTER   IX 

SEED  PLANTS  (SPERMATOPHYTA)  (GYMNO SPERM 'JET) 

ONE  of  the  most  notable  peculiarities  of  the  higher 
Pteridophytes  is  the  extreme  reduction  of  the  gameto- 
phyte  and  the  corresponding  specialization  of  the 
sporophyte.  This  culminates  in  the  various  hetero- 
sporous  types,  where  the  gametophyte  may  lack  all 
power  of  independent  growth  and  serve  merely  to  de- 
velop the  reproductive  organs  and  nourish  the  embryo- 
sporophyte  until  it  is  self-supporting.  In  Selaginella 
the  gametophyte  is  partially  developed  within  the 
spores  while  they  are  still  included  within  the  sporan- 
gium, and  is  nourished  directly  from  the  sporophyte 
through  the  sporangium  wall,  which  serves  thus  not 
only  to  protect  the  spores,  but  also  to  nourish  them 
during  the  early  stages  of  germination.  Finally,  how- 
ever, the  spores  are  discharged  from  the  sporangium, 
and  the  gametophyte  completes  its  development  away 
from  the  sporophyte. 

In  the  highest  of  all  plants,  the  seed-bearing,  or, 
as  they  are  commonly  called,  the  "flowering  plants," 
heterospory  is  carried  one  step  further,  and  the  macro- 
spore  remains  permanently  within  the  sporangium.  Not 
only  is  the  germination  of  the  spore  completed  within  the 
sporangium,  but  the  fertilization  of  the  archegonium  is 
effected  and  the  development  of  the  embryo-sporophyte 

168 


SEED  PLANTS  159 

is  begun.  The  microspores,  however,  although  the 
germination  begins  within  the  sporangium,  are  finally 
discharged  and  complete  their  development  outside 
the  sporangium,  precisely  as  in  the  Pteridophytes. 

The  Spermatophytes  do  not  differ  in  any  essential 
structural  points  from  the  Pteridophytes.  Like  them 
they  produce  sporangia,  usually  upon  special  leaves 
(sporophylls),  which  are  here  known  as  carpels  and 
stamens.  Upon  the  former  are  borne  macrosporangia 
(ovules),  upon  the  latter  the  microsporangia  (pollen- 
sacs).  These  sporangia  agree  closely  in  their  structure 
and  development  with  those  of  the  higher  Pteridophytes. 
In  the  microsporangium  the  development  of  the  spores 
(pollen)  corresponds  in  the  minutest  particulars  with 
that  of  the  microspores  of  the  heterosporous  Pterido- 
phytes, but  in  the  macrosporangium,  especially  in  the 
higher  Spermatophytes,  the  Angiosperms,  there  is  not 
always  the  division  of  the  spore  mother-cell  into  four 
daughter-spores.  The  macrospore  in  these  forms  is  usu- 
ally known  as  the  "  embryo-sac." 

The  development  of  the  female  gametophyte  within 
the  embryo-sac,  especially  in  the  lower  types  (Gym- 
nosperms),  agrees  very  closely  with  that  in  Selaginella 
and  Isoetes.  After  the  germination  is  complete  and  the 
embryo  has  developed  from  the  fertilized  egg-cell  of 
the  archegonium,  the  wall  of  the  macrosporangium 
hardens  and  forms  a  firm  protective  covering  for  the 
enclosed  embryo,  which  generally  is  imbedded  in  the 
tissue  of  the  gametophyte.  The  latter  becomes  filled 
with  food-substances,  such  as  oil,  starch,  and  nitrogenous 
compounds,  for  the  future  growth  of  the  embryo.  The 
sporangium  now  falls  away  from  the  sporophyte,  and  is 


160  EVOLUTION  OF   PLANTS 

known  as  a  seed.  This  peculiar  modification  of  the 
macrosporangium  to  form  a  seed  is  the  real  distinguish- 
ing characteristic  of  the  Spermatophytes. 

The  microspores  or  pollen-spores  of  the  seed  plants 
differ  very  little  from  those  of  the  ferns  either  in  form 
or  development,  and  indeed  are  strictly  homologous  with 
the  spores  of  all  Archegoniates,  where,  as  we  have  seen, 
the  spores  invariably  arise  from  the  division  of  the 
sporogenous  cell  into  four  equal  parts. 

Owing  to  the  position  of  the  archegonium  within  the 
macrosporangium,  the  method  of  fertilization  is  different 
from  that  in  the  Pteridophytes,  where  the  free  gameto- 
phytes  are  directly  exposed  to  the  action  of  water,  and 
motile  spermatozoids  are  produced  in  the  antheridium. 
The  pollen-spore  of  the  Spermatophytes  on  germination 
produces  a  long  tubular  filament  within  which  is  con- 
tained the  very  rudimentary  antheridium  with  usually 
two  sperm-cells.  In  its  growth  the  pollen-tube  grows 
down  through  the  tissues  above  the  apex  of  the  female 
gametophyte,  and  finally  reaches  the  archegonium,  where 
it  discharges  the  sperm-cells,  one  of  which  fuses  with 
the  egg-cell,  thus  effecting  fecundation.  Until  very 
recently  it  was  supposed  that  the  absence  of  motile 
spermatozoids  formed  an  absolute  distinction  between 
Pteridophytes  and  Spermatophytes,  but  the  discovery  of 
large  fern-like  spermatozoids  in  certain  Cycads,  as  well 
as  in  the  curious  genus  Gingko,  has  broken  down  the 
last  barrier  between  the  two  groups. 

The  "  flower  "  in  most  Spermatophytes  is  a  collection 
of  sporophylls,  or  spore-bearing  leaves,  the  carpels, 
bearing  macrosporangia  (ovules),  and  the  stamens,  bear- 
ing the  microsporangia  (pollen-sacs).  These  sporophylls 


SEED  PLANTS  161 

may  be  compared  directly  with  those  of  the  Pterido- 
phytes,  which  are  sometimes  grouped  in  a  spike  or 
strobilus,  as  is  seen  in  the  horsetails  and  club-mosses, 
and  this  strobilus  is  structurally  much  like  the  flower  of 
some  of  the  lower  Spermatophytes,  especially  the  Conif- 
erse. 

In  the  Cycads,  which  are  the  lowest  known  Spermato- 
phytes, the  foliar  nature  of  the  sporophylls  is  very 
obvious  (Fig.  40,  A),  but  in  the  higher  forms  this  is  not 
usually  so  evident,  especially  as  regards  the  carpels. 
In  addition  to  the  sporophylls,  most  of  the  higher  Sper- 
matophytes have  accessory  floral  leaves,  sepals  and 
petals,  which,  however,  are  by  no  means  necessarily 
present. 

The  seed-bearing  plants  are  commonly  divided  into 
two  great  divisions,  Gymnosperms  and  Angiosperms. 
The  former,  which  include  the  Cycads,  the  Conifers,  and 
a  third  less  familiar  order,  the  Gnetacese,  or  "  joint-firs," 
are  characterized  by  having  the  macrosporangium  borne 
upon  an  open  carpellary  leaf;  hence  the  name,  Gymno- 
spermse,  or  naked-seeded  plants.  In  the  Angiosperms, 
the  second  group,  the  carpel  (or  carpels)  forms  a  closed 
cavity,  the  ovary,  in  which  the  ovules,  and  later  the 
seeds,  are  completely  enclosed.  It  is  this  last  group 
which  comprises  the  vast  majority  of  the  flowering 
plants. 


162  EVOLUTION  OF  PLANTS 

THE  GYMNOSPERM.E 

In  the  Gymnosperms  the  flowers  are  of  the  simplest 
character,  consisting  entirely  of  sporophylls  of  one 
kind.  Macrospores  and  microspores  are  always  borne 
in  different  flowers  and  very  often  upon  different  plants. 

THE  CYCADS  (Cycadacece) 

Without  question  the  lowest  types  of  seed-bearing 
plants  known  are  the  Cycadacese,  a  group  of  palm-like 
plants  of  which  the  best  known  is  the  so-called  "  sago- 
palm"  of  the  greenhouses,  Cycas  revoluta.  About  sev- 
enty-five living  species  of  Cycads  are  known,  widely  dis- 
tributed through  the  warmer  regions  of  both  the  Old  and 
the  New  worlds.  Most  of  them  are  strictly  tropical,  but 
one  species,  Zamia  integrifolia,  is  found  as  far  north  as 
Florida,  and  Cycas  revoluta  probably  extends  beyond  the 
northern  tropic  in  Japan.  They  recall  in  many  ways 
certain  ferns,  and  a  careful  examination  of  the  tissues 
of  the  sporophyte  shows  that  these  resemblances  are 
more  than  superficial.  The  tissues  of  the  fern-like 
leaves  resemble  those  of  the  lower  ferns,  and  the  leaves 
when  young  are  coiled  up  much  as  in  the  ordinary  ferns 
(Fig.  40,  F).  The  plant,  however,  may  develop  a 
primary  tap-root  like  that  of  the  Conifers  or  Dicotyle- 
dons, and  there  is  a  more  or  less  marked  secondary 
thickening  of  the  vascular  bundles  of  the  stem,  which, 
however,  also  occurs  in  a  few  ferns. 

In  Cycas  the  macrosporangia  are  borne  upon  leaves 
which  differ  but  slightly  from  the  ordinary  ones  (Fig. 
40,  A).  The  sporangia  are  very  large,  sometimes  being 


SEED   PLANTS 


163 


of  the  size  of  a  large  plum.  The  very  large  macrospore 
(Fig.  40,  B,  ma)  has  a  definite  thick  membrane  like  that  of 
the  ferns,  but  is  retained  permanently  within  the  sporan- 
gium. So  far  as  it  is  known,  the  development  of  the 


FIG.  40  (Cycadacese) . — A,  a  sporophyll  of  Cycas  circinalis,  with  six 
ovules  (macrosporangia) ,  ma  ;  B,  longitudinal  section  of  a  young  ovule 
of  C.  revoluta,  showing  the  single  large  macrospore,  ma  ;  C,  a  sporophyll 
from  the  male  cone  of  C.  revoluta,  showing  the  lower  surface  covered 
with  groups  or  sori  of  microsporangia,  mi;  D,  a  single  sorus  of  five 
microsporangia ;  E,  a  microspore  (pollen-spore),  showing  the  rudimen- 
tary antheridium,  cm;  the  larger  antheridial  cell  later  gives  rise  to 
two, large  spermatozoids ;  F,  a  young  leaf  of  C.  revoluta,  showing  the 
fern-like  coiling  of  the  divisions ;  G,  a  scale  from  the  female  cone  of 
Zamia  integrifolia,  with  two  ovules,  ma;  H,  section  through  the  ovule 
at  the  time  of  fertilization ;  pc,  the  pollen-chamber  with  three  germi- 
nating pollen-spores ;  y,  the  vegetative  tissue  of  the  female  gametophyte 
contained  within  the  macrospore;  ar,  two  archegonia;  I,  a  spermato- 
zoid  of  Zamia,  showing  the  numerous  cilia,  c.  (Figs.  H,  I,  after  Webber.) 


gametophyte  is  much  like  that  of  Isoetes  or  Selaginella, 
but  the  details  are  still  somewhat  imperfectly  known. 
The  gametophyte,  if  fertilization  is  not  effected,  may 
grow  out  beyond  the  spore  and  develop  chlorophyll, 


164  EVOLUTION  OF  PLANTS 

and  thus  is  capable  of  a  certain  degree  of  independent 
existence,  a  condition  not  known  in  any  other  Spermato- 
phytes.  The  several  archegonia  produced  upon  the 
gametophyte  do  not  differ  in  any  essential  particular 
from  those  of  the  true  Archegoniates. 

The  microsporangia  occur  in  great  numbers  upon  the 
backs  of  sporophylls  which  are  arranged  spirally  about 
a  thick  axis  and  form  a  cone  or  strobilus.  The  micro- 
sporangia  are  very  much  like  those  of  the  ferns,  and 
are  usually  grouped  in  clusters  or  sori  (Fig.  40,  C,  D). 
The  microspore  on  germinating  produces  a  rudimentary 
plant  with  a  simple  antheridium  containing  two  sperm- 
cells.  From  these  are  produced  the  spermatozoids, 
much  like  those  of  the  ferns,  but,  especially  in  Zamia, 
enormously  larger  than  any  other  known  spermato- 
zoids. These  are  formed  shortly  before  fertilization 
takes  place. 

The  pollen  falls  upon  the  top  of  the  ovule  (macro- 
sporangium),  where  there  is  an  opening  in  the  integu- 
ment with  which  it  is  surrounded,  and  this  opening  at 
the  time  of  pollination  is  filled  with  a  fluid  which  on 
evaporating  deposits  the  pollen-spores  upon  the  top  of 
the  sporangium  itself,  where  they  germinate  by  sending 
out  the  pollen-tube,  which  forces  its  way  through  the 
upper  part  of  the  ovule  to  a  cavity  just  above  the  arche- 
gonium  (Fig.  40,  H).  Simultaneously  with  the  ripening 
of  the  latter,  the  two  spermatozoids  within  the  pollen- 
tube  are  discharged  into  the  cavity,  which  is  filled  with 
a  watery  fluid  derived  from  the  distended  pollen-tubes, 
and  in  this  they  swim  to  the  archegonium  by  means  of 
the  numerous  cilia  with  which  they  are  furnished.  Fer- 
tilization is  thus  effected  precisely  as  in  the  Arche- 


SEED   PLANTS 


165 


goniatse,  and  the  egg  thereupon   begins   to  grow  and 
develops  into    the    embryo-sporophyte,   while    the   sur- 
rounding cells  of  the  gametophyte  become  filled  with 
food-materials        and 
are     known    as    the 
"  endosperm."       The 
wall    of   the    sporan- 
gium   now    hardens,          ^^™^^     «;—  // 
while  the  outer  tissues  ^2Sln     .m  $5)    rl-sc 

of  the  integument 
become  pulpy,  so  that 
the  ripened  seed  looks 
very  much  like  the 
fleshy  fruit  of  a  plum 
or  cherry. 

That  the  Cycads 
represent  a  very  an- 
cient type  is  shown 
by  their  fossil  re- 
mains, which  indicate 
that  during  the  Mes- 
ozoic  age  they  were 
among  the  most  abun- 
dant plants.  They 
occurred  in  great  num- 
bers, and  comprised 
many  more  genera 
and  species,  as  well 
as 


FIG.  41  (Coniferae).  —  A,  a  branch  of  a  fe- 
male plant  of  the  common  yew  (Taxus) , 
one  of  the  simplest  Conifers ;  ma,  young 
female  flower;  fr,  ripe  fruit ;  B,  a  single 
female  flower,  consisting  of  an  ovule,  or 
macrosporangium,  ma,  surrounded  by  a 
number  of  scale-leaves  ;  C,  a  section  of 
the  flower,  showing  the  terminal  spo- 
rangium (ovule) ,  m,  surrounded  by  the 
integument,  in,  and  the  scales,  .sc;  D, 
section  of  an  older  ovule,  showing  the 
large  macrospore  ("  embryo-sac  "),  sp  ; 
E,  the  ripe  fruit,  with  one  side  of  the 
cup-shaped  aril,  ar,  cut  away  to  show 
the  seed,  s ;  the  seed  is  the  matured 
ovule,  the  aril  a  special  structure  which 
grows  up  about  the  seed ;  F,  a  male 
flower  of  Taxus,  showing  the  umbrella- 
shaped  sporophylls,  each  bearing  sev- 
eral microsporangia  upon  the  lower 
surface ;  G,  a  single  sporophyll ;  mi, 
the  sporangia;  H,  a  leaf  of  Gingko, 
showing  the  fern-like  form  and  vena- 
tion. (Figs.  F,  G,  after  Eichler.) 

individuals,    than 
at   present.     The   first   evidences  of   the   existence    of 
Cycads  occur  in  the  Carboniferous  rocks,  but  in  small 
numbers ;  but  in  the  Mesozoic  rocks,  as  already  stated, 


166  EVOLUTION  OF  PLANTS 

they  occur  in  great  numbers.  The  oldest  forms  closely 
resembled  the  existing  genus  Cycas,  which  has  persisted 
while  many  of  the  more  specialized  types  have  become 
quite  extinct. 

Perhaps  allied  to  the  Cycads,  and  like  them  also 
a  very  old  type,  is  the  curious  genus  Gingko  (Fig. 
41,  H),  represented  at  present  by  a  single  species  no 
longer  known  in  a  wild  state,  but  much  planted  about 
temples  in  China  and  Japan,  where  gigantic  trees,  hun- 
dreds of  years  old,  are  standing.  From  the  fern-like 
venation  of  the  leaves,  the  tree  is  sometimes  called  the 
maiden -hair  tree,  and  this  peculiarity  of  the  leaves  prob- 
ably indicates  a  real  affinity  with  the  ferns.  Many  fossil 
species,  much  like  the  existing  one,  are  known,  the  oldest 
ones  from  the  Permian  rocks,  and  therefore  somewhat 
more  recent  than  the  oldest  Cycads. 

Gingko  is  usually  referred  to  the  Coniferse,  but  the 
development  of  the  gametophyte,  especially  th*e  produc- 
tion of  multiciliate  sperrnatozoids  like  those  of  Cycas, 
as  well  as  the  fern-like  character  of  the  leaves,  suggest 
that  its  affinities  are  rather  with  the  Cycads  than  with 
the  Conifers. 

THE  CONIFERS 

Although  the  Cycads  and  Coniferse  are  usually  asso- 
ciated in  a  common  group,  Gymnospermse,  it  is  at  least 
doubtful  in  view  of  the  recent  discoveries  in  regard  to 
the  former,  as  well  as  because  of  other  differences  in 
structure,  whether  these  two  orders  are  really  related  to 
each  other. 

The  Coniferse  are  the  familiar  "  evergreen  "  trees  of 


SEED  PLANTS  167 

our  northern  forests,  and  while  a  much  more  ancient 
type  than  the  Angiosperms,  they  are  still  a  predominant 
type  of  vegetation  in  many  regions,  where  the  forests  are 
often  composed  almost  exclusively  of  these  trees.  In 
contrast  to  the  Cycads,  which  rarely  attain  tree-like 
proportions,  and  whose  leaves  are  large  and  fern-like, 
the  Conifers  usually  become  trees,  often  of  gigantic  size, 
and  in  most  of  them  the  leaves  are  small  and  needle- 
shaped.  In  the  relation  of  stem  and  leaves  the  Coni- 
fers recall  the  club-mosses,  while  the  Cycads  are  very 
much  like  the  ferns,  and  it  is  not  impossible  that  this 
may  indicate  an  entirely  independent  origin  for  the 
two  groups  from  Lycopods  and  ferns  respectively.  The 
recurrence  of  fossil  forms  of  an  intermediate  character 
supports  such  a  hypothesis. 

The  sporophyte  in  the  Conifers,  as  already  stated,  is 
always  large,  usually  becoming  arborescent,  and  some- 
times a  hundred  metres  and  more  in  height.  These 
giant  trees  reach  their  greatest  development  on  the 
western  slopes  of  the  mountains  of  Pacific  North 
America,  where  a  number  of  species  attain  a  height  of 
one  hundred  metres,  or  it  is  claimed  one  hundred  and 
fifty  metres,  with  trunks  from  five  to  six  metres  in 
diameter;  or,  in  the  case  of  the  great  Calif ornian 
Sequoias,  ten  metres  or  even  more.  The  extraordi- 
nary height  of  coniferous  trees,  which  almost  always 
exceeds  that  of  their  deciduous  companions,  is  due  to 
the  persistence  of  the  original  apical  bud,  which,  unless 
injured  by  accident,  remains  active,  so  that  a  definite 
central  axis  is  formed  which  may  grow  in  length  for 
hundreds  of  years.  The  regular  whorls  of  branches 
formed  at  the  base  of  each  year's  growth  in  many  spe- 


168  EVOLUTION  OF  PLANTS 

cies  gives  them  the  strikingly  symmetrical  conical  form 
so  characteristic  of  most  of  the  group. 

The  leaves  of  the  Conifers  are  usually  slender 
"needles,"  or  are  small  and  scale-like,  as  in  the  cypress 
and  arbor-vitse.  Usually  they  remain  attached  to  the 
stem  for  several  years,  but  in  a  few  cases,  like  the  larch 
and  bald  cypress,  they  are  shed  annually.  Like  the 
Cycads,  the  Conifers  generally  have  a  main  tap-root, 
which,  like  the  stem,  shows  a  continuous  secondary 
growth  in  thickness.  This  in  the  stem  results  in  the 
formation  of  the  well-known  annual  growth-rings.  This 
secondary  growth  is  much  like  that  found  in  the  stems 
of  normal  Dicotyledons,  and  on  the  strength  of  this  the 
older  botanists  united  these  with  the  Gymnosperms 
under  the  name  "  Exogens  " ;  but  the  great  differences 
in  the  structure  of  the  flower,  and  especially  in  the 
gametophyte,  forbid  the  idea  df  such  a  union,  and 
botanists  are  now  agreed  that  no  near  relationship  exists 
between  the  two. 

The  flowers  of  the  Coniferse  are  very  simple  in  struct- 
ure. In  the  lowest  types,  like  the  yew  (Taxus)  (Fig. 
41,  A-G),  the  macrosporangium  is  borne  directly  at  the 
end  of  a  shoot,  and  is  in  fact  its  transformed  apex.  It 
becomes  invested  with  an  integument  like  that  found 
in  Cycas,  and  is  protected  while  young  by  several  over- 
lapping scale-leaves.  Within  is  produced  a  group  of 
sporogenous  cells,  from  one  of  which  is  developed  the 
single  macrospore  which  gives  rise  to  a  gametophyte 
of  considerable  size  with  several  archegonia.  The 
microsporangia  are  formed,  several  together,  upon  um- 
brella-shaped leaves,  which  are  arranged  in  a  cone  which 
suggests  that  of  Equisetum  (F,  G).  The  germinating 


SEED   PLANTS  169 

microspore  produces  a  rudimentary  antheridium  with 
two  sperm-cells,  much  as  in  Cycas,  but  so  far  as  known 
at  present,  the  formation  of  spermatozoids  is  completely 
suppressed.  It  would  not  be  surprising,  however,  if 
some  trace  of  such  structures  should  be  discovered. 
The  germination  of  the  pollen-spore  when  it  falls  upon 
the  ovule  is  like  that  of  Cycas,  but  the  pollen-tube  pene- 
trates through  the  neck  of  the  archegonium,  and  the 
sperm-nucleus  is  discharged  directly  into  the  egg. 

In  the  higher  Conifers,  such  as  the  pines  and  firs,  the 
macrosporangia  are  developed  upon  special  sporophylls 
(carpels)  which  not  infrequently  are  borne  in  the  axils 
of  sterile  bracts.  The  sporophylls  are  arranged  spirally 
about  the  axis  of  a  shoot,  forming  the  familiar  "  cones  " 
of  these  trees.  As  in  the  yew,  the  single  macrospore 
which  is  finally  formed  in  the  sporangium  produces 
the  large  female  gametophyte,  much  like  that  of  Sela- 
ginella.  The  archegonia  are  several  in  number,  with 
very  large  egg-cells,  but  the  neck  parts  relatively  small, 
as  they  are  in  all  Gymnosperms.  The  sporangium  is 
invested  with  a  single  integument  as  in  the  other  forms 
described. 

The  microsporangia  are  also  borne  upon  special  sporo- 
phylls, and  are  usually  arranged  in  a  cone  like  those 
bearing  the  ovules.  These  microsporangia  or  pollen- 
sacs  correspond  in  every  detail  of  their  development 
with  those  of  the  Pteridophytes.  The  ripe  pollen- 
spores  in  the  pines  (Fig,  42,  E)  and  firs  are  provided 
with  wing-like  outgrowths  of  the  outer  membrane, 
which  form  very  efficient  sails  by  which  they  are  more 
easily  scattered  by  the  wind.  As  the  pollen  must  de- 
pend upon  the  wind  for  its  distribution,  the  number  of 


170 


EVOLUTION   OF   tLANTS 


pollen-spores  produced  is  enormously  in  excess  of  the 
macrospores.  Indeed,  so,  abundant  is  the  pollen,  that 
the  ground  in  the  neighborhood  of  the  trees  is  some- 


FIG.  42  (Coniferse) .  —  A,  branch  of  a  pine  (Pinus  contorta)  with  male 
flowers,  fl ;  B,  longitudinal  section  of  a  single  flower,  showing  the 
arrangement  of  the  sporophylls;  C,  a  single  sporophyll,  showing  the 
two  microsporangia,  mi,  upon  its  lower  surface ;  D,  a  section  through 
the  microsporangium  ;  E,  a  single  microspore,  showing  the  antheridium, 
an,  and  the  vesicular  outgrowths  of  the  wall,  v,  which  serve  as  sails ; 
F,  a  female  flower  of  the  same  pine;  G,  a  single  sporophyll  from  the 
female  flower,  showing  the  small  scale,  sc,  by  which  it  is  subtended  ;  H, 
a  sporophyll  from  an  older  cone,  showing  two  macrosporangia  (ovules), 
ma,  upon  its  inner  face;  I,  longitudinal  section  of  an  ovule  (macro- 
sporangium)  ;  the  large  macrospore  contains  the  gametophyte,  g, 
bearing  several  archegonia,  ar;  p,  a  pollen-spore  sending  down  the  tube 
by  which  the  archegonia  are  fertilized ;  J,  a  young  embryo ;  sus,  sus- 
penspr ;  x,  apical  cell ;  K,  section  of  a  ripe  seed,  containing  the  embryo, 
em,  imbedded  in  the  prothallial  tissue,  g ;  L,  young  sporophyte,  show- 
ing the  cotyledons,  cot;  stem,  st;  root,  r;  s,  the  empty  seed-coat. 


times  covered   with    a    layer   of    the    sulphur-colored 
powder. 

The  germination  of  the  pollen-spores  and  the  fertili- 
zation of  the  archegonium  are  effected  as  in  Taxus.  As 
in  that  genus,  no  trace  of  motile  spermatozoids  has  yet 


SEED    PLANTS  171 

been  found  in  the  higher  Coniferse,  and  it  is  not  likely 
that  in  these  sperraatozoids  exist. 

The  development  of  the  embryo  in  the  Conifers  shows 
a  good  deal  of  difference  in  different  genera.  Some- 
times but  a  single  embryo  arises  from  each  egg-cell,  as 
in  most  other  plants ;  but  sometimes,  for  instance  in  the 
common  pines  and  firs,  each  egg  gives  rise  to  a  group 
of  (usually  four)  embryos,  and  the  ripe  seed  may  contain 
more  than  one  young  sporophyte.  Generally,  however, 
one  of  the  growing  embryos  crowds  out  the  others,  and 
only  this  one  matures.  As  in  Selaginella  and  the  lower 
Gymnosperms,  a  long  suspensor  (Fig.  42,  J,  sus)  is 
formed  from  the  upper  part  of  the  egg,  while  the  lower 
portion  only  gives  rise  to  the  embryo  itself.  By  the 
rapid  lengthening  of  the  suspensor  the  growing  embryo 
is  pushed  down  into  the  tissue  of  the  gametophyte, 
whose  cells  become  gradually  filled  with  nutrient  sub- 
stances upon  which  the  developing  embryo  feeds.  These 
are  not  all  consumed,  however,  but  a  considerable  part 
persists  in  the  ripe  seed  as  the  "  endosperm,"  in  which 
the  young  sporophyte  is  imbedded,  and  upon  which  it 
draws  for  nourishment  in  the  early  stages  of  the  germi- 
nation of  the  seed.  The  young  sporophyte  within  the 
ripe  seed  already  has  all  its  primary  organs  developed. 
The  stem  is  prolonged  downward  into  the  primary  root, 
which  is  directed  toward  the  opening  in  the  integument 
(micropyle),  while  the  upper  end  of  the  embryo  termi- 
nates in  the  conical  stem-apex  about  which  is  arranged 
a  circle  of  primary  leaves,  or  cotyledons,  ranging  in 
number  from  two  to  half  a  dozen  or  more. 

The  ripe  seed  has  a  hard,  usually  dark-colored  coat, 
effectually  protecting  the  delicate  inner  tissues;  some- 


172  EVOLUTION   OF  PLANTS 

times  attached  to  it  are  membranaceous  wings  to  facili- 
tate its  distribution  by  the  wind. 

Before  the  seed  germinates,  the  enclosed  sporophyte 
absorbs  water  rapidly,  and  the  dormant  protoplasm  of 
its  cells  resumes  its  activity.  The  little  plant  increases 
quickly  in  size,  growing  at  the  expense  of  the  food 
stored  in  the  surrounding  endosperm.  The  root  elon- 
gates, and  pushes  out  through  the  micropyle,  turns 
downward,  and  buries  itself  in  the  earth,  and  thus  fas- 
tens the  young  sporophyte  into  the  ground.  In  the 
meantime  the  cotyledons  have  enlarged  and  turned 
green,  and  finally  pull  themselves  out  of  the'seed,  whose 
empty  shell  is  thrown  aside.  The  young  sporophyte  is 
now  quite  independent,  and  in  course  of  time  assumes 
its  perfect  form. 

The  stem  of  the  seedling  sporopl^te  contains  a  circle 
of  separate  vascular  bundles,  not  unlike  those  in  the 
stems  of  some  Pteridophytes,  but  there  is  soon  devel- 
oped in  each  bundle  a  zone  of  growing  tissue,  finally 
connected  with  that  of  the  other  bundles  by  means  of 
a  similar  zone  developed  in  the  tissue  lying  between 
the  separate  bundles.  This  zone  of  growing  tissue,  or 
"  cambium,"  characterizes  the  older  stems  of  all  Conifers, 
and  to  its  continued  activity  is  due  the  annual  growth- 
rings  found  in  these  trees.  A  similar  secondary  growth 
in  thickness  is  known  to  have  taken  place  in  the  stems 
of  the  fossil  Lepidodendrons,  which  it  has  been  suggested 
may  have  been  the  progenitors  of  the  modern  Coniferge. 

Unless  injured,  the  original  stem-apex  of  the  embryo 
persists  in  the  older  sporophyte,  and  to  this  is  due,  as 
we  have  said,  the  extraordinary  height  which  some  of 
the  Conifers  attain. 


SEED   PLANTS  173 

THE  GNETACE^E 

< 

The  last  order  of  the  Gymnosperms,  the  Gnetaceae, 
are  forms  familiar  only  to  the  botanist,  the  only  exam- 
ples occurring  in  the  United  States  being  a  few  species 
of  Ephedra  in  the  deserts  of  the  Southwest.  The  other 
two  genera  are  strictly  tropical.  It  is  a  question  how 
closely  the  three  genera  are  related,  as  they  differ  very 
much  from  one  another,  as  well  as  from  the  other  Gym- 
nosperms. Some  of  them  show  certain  analogies  with 
the  Dicotyledons,  and  they  are  sometimes  regarded  as 
forms  connecting  the  Gymnosperms  with  the  latter. 
Their  development  is  not  known  with  sufficient  com- 
pleteness, however,  to  make  this  at  all  certain,  and  the 
few  fossil  remains  attributed  to  this  order  are  much  too 
imperfect  to  throw  much  light  upon  their  geological 
history. 

FOSSIL  CONIFERS 

Most  of  the  living  genera  of  Conifers  are  also  found 
fossil,  and  some  of  them  which  are  now  restricted  to  a 
very  limited  area  were  evidently  much  more  widespread 
in  earlier  geological  times.  None  of  the  living  genera 
can  be  traced  with  certainty  further  back  than  the  earlier 
Mesozoic  rocks,  although  a  number  of  fossils  from  the 
coal  measures  have  been  doubtfully  assigned  to  existing 
genera.  In  the  later  Mesozoic  and  early  Tertiary  rocks, 
however,  there  are  abundant  evidences  of  the  existence 
of  many  living  genera,  or,  in  a  few  instances,  even  spe- 
cies. A  notable  case  is  that  of  the  genus  Taxodium,  with 
two  existing  species  in  the  southeastern  United  States 
and  Mexico.  Of  these,  the  common  bald  cypress  of  the 


174  EVOLUTION   OF   PLANTS 

Gulf  States  is  represented  in  Tertiary  deposits  by  an  ap- 
parently identical  species,  which  at  that  time  had  a  wide 
range  over  nearly  the  whole  northern  hemisphere.  The 
genus  Sequoia  is  another  striking  example  of  the  sur- 
vival, in  a  limited  range,  of  a  once  widely  distributed 
type.  At  present  the  two  species,  S.  sempervirens,  the 
coast  redwood  of  California,  and  S.  gigantea,  the  giant 
tree  of  the  Sierra  Nevada,  are  all  that  remain  of  a 
genus  once  represented  by  numerous  widely  distributed 
species. 

Besides  the  genera  still  existing  there  are  a  number 
known  only  as  fossils,  some  of  which  extend  back  to 
the  Carboniferous.  The  exact  relation  of  these  extinct 
forms  to  the  existing  Coniferse  is  somewhat  doubtful. 


THE  CORDAITE^: 

Probably  allied  to  the  Conifers  is  a  peculiar  group  of 
fossils,  the  Cordaiteae.  These  first  appear  in  very  old 
formations,  some  writers  claiming  that  they  are  found 
in  the  Silurian  rocks.  The  occurrence  of  seed-bearing 
plants  in  such  ancient  formations  is,  to  say  the  least, 
unexpected.  They  are  most  abundant  in  the  coal  meas- 
ures and  disappear  soon  after.  The  flowers  have  been 
preserved  in  some  instances  in  an  astonishingly  perfect 
condition,  even  the  pollen-spores  with  an  enclosed  struct- 
ure supposed  to  be  the  gametophyte  being  recogniza- 
ble. The  latter  is  much  more  highly  developed  than  in 
any  living  seed  plants,  and  this  shows  the  primitive 
nature  of  these  plants.  In  regard  to  the  structure  of 
the  flowers,  they  show  certain  resemblances  to  both 


SEED    PLANTS  175 

Cycads  and  Conifers,  but  on  the  whole  they  are  prob- 
ably nearer  the  latter. 


SUMMARY 

Compared  with  the  Angiosperms,  the  Gymnosperms 
are  an  ancient  primitive  group  of  plants  showing  very 
evident  resemblances  to  the  Pteridophytes,  from  which 
they  have  doubtless  originated.  It  is  not  likely  that  the 
existing  Gymnosperms  constitute  a  homogeneous  class, 
but  it  is  more  probable  that  they  are  remnants  of  at 
least  three  lines  of  development.  Of  these  the  Cycads 
show  evident  relationships  to  the  ferns,  this  being 
evinced  by  the  character  of  the  leaf  and  flowers,  and 
still  more  by  the  form  of  the  spermatozoids.  The  geo- 
logical record  shows  that  the  Cycads  were  once  a  much 
more  important  group  than  at  present.  Perhaps  related 
to  these  is  the  genus  Gingko,  also  a  very  old  type,  with 
but  a  single  living  representative. 

The  Conifers  are  a  more  recent  type  than  the  Cycads, 
but  still  are  older  than  the  Angiosperms.  In  the  gen- 
eral habit  of  the  sporophyte,  especially  the  sporophylls, 
they  suggest  a  direct  connection  with  the  Lycopods, 
and  this  is  borne  out  by  a  study  of  the  gametophyte, 
which  closely  resembles  that  of  Selaginella.  The  pres- 
ence in  the  Carboniferous  rocks  of  gigantic  Lycopods 
now  extinct,  suggests  these  as  possible  ancestors  for  the 
existing  Conifers. 

There  is  very  little  in  common  between  the  third 
order,  the  Gnetaceae,  and  the  other  Gymnosperms,  and 
it  is  questionable  whether  the  theory  that  the  Gnetacere 


176  EVOLUTION  OF  PLANTS 

are  forms  intermediate  between  the  other  Gymnosperms 
and  the  Angiosperms  will  prove  to  be  correct.  It  seems 
quite  as  likely  that  the  latter  have  originated  indepen- 
dently directly  from  the  Pteridophytes,  or  possibly  from 
low  forms  allied  to  the  Cycads. 


CHAPTER  X 

ANGIOSPEEM^E   (MONOCOTYLEDONS} 

THE  second  great  division  of  the  seed-bearing  plants, 
the  Angiosperms,  is  preeminently  the  prevailing  modem 
plant  type.  These  are  the  plants  ordinarily  thought  of 
as  "flowering  plants."  They  are  at  once  distinguished 
from  the  Gymnosperms  by  the  development  of  a  closed 
ovary  formed  from  the  carpel,  or  by  the  union  of  two 
or  more  carpels.  Within  this  closed  cavity  are  borne 
the  ovules  or  macrosporangia,  which  are  usually,  but 
not  always,  outgrowths  of  the  carpellary  leaves.  Some- 
times the  apex  of  the  floral  axis  or  shoot  is  transformed 
directly  into  the  ovule. 

The  flowers  of  the  Angiosperms  exhibit  extraordi- 
nary variety,  and  contrast  strongly  with  the  very  uni- 
form character  of  the  flowers  of  most  Gymnosperms. 
In  the  simplest  types  (Fig.  43)  the  flowers  of  the 
Angiosperms  are  nearly  as  simple  as  the  simplest 
Gymnosperms,  but  as  a  rule  they  are  far  more  complex. 
This  arises  primarily  from  a  multiplication  of  the 
sporophylls,  but  is  further  complicated  by  the  develop- 
ment of  accessory  leaves,  sepals  and  petals,  never  found 
in  the  Gymnosperms. 

In  most  Angiosperms  both  sorts  of  sporophylls  are 
usually  associated  in  the  same  flower ;  i.e.  the  flower 
contains  both  carpels  and  stamens,  which  are  sur- 
N  177 


178 


EVOLUTION   OF   PLANTS 


rounded  by  the  floral  envelopes,  corolla  and  calyx, 
made  up  respectively  of  the  petals  and  sepals.  The 
base  of  the  carpel  forms  the  ovary,  while  above  it  is 

prolonged  into  the  style  tipped 
by  the  stigma,  or  portion  upon 
which  the  pollen  falls  (Fig.  44, 
A). 

In  the  number  of  parts  in  the 
flower,  as  well  as  in  their  ar- 
rangement and  form,  the  Angio- 
sperms  show  almost  infinite 
variety.  The  petals  are  very 
frequently  brightly  colored,  and 
this,  together  with  many  modi- 
fications in  the  other  structures, 
is  intimately  associated  with 
pollination  through  insect  aid, 
which  has  undoubtedly  played 
an  important  part  in  the  evolu- 
tion of  the  floral  structures  of 
the  Angiosperms. 

The  gametophyte  in  the  An- 
giosperms is  so  much  reduced 
and  so  inconspicuous  that  it  is 
usually  quite  ignored  in  the 
ordinary  study  of  these  plants ; 
but  it  must  be  borne  in  mind 
that  the  gametophyte  is  always 
present,  although  in  a  very  re- 
duced form.  As.  in  the  Gymnosperms,  the  ovule  cor- 
responds to  the  macrosporangium  of  the  heterosporous 
Pteridophytes,  and  within  it  is  formed  the  single  macro- 


flower,  A,  and  the  female, 
B,  are  much  alike.  Each 
consists  of  a  single  sporan- 
gium invested  with  an  in- 
tegument, in,  the  whole 
enclosed  in  a  tubular  leaf 
with  spiny  processes  at  the 
summit.  This 


section  through  the  base  of 
the  female  flower,  with  the 
enclosed  macrosporangi- 
um, or  ovule,  ma,  and  two 
integuments,  in,  and  the 
contained  embryo-sac,  or 
macrospore,  sp. 


ANGIOSPERM^E 


179 


spore  or  embryo-sac  (Fig.  44,  A,  ma,  B),  which  in  its 
origin  corresponds  closely  to  that  in  the  Gymriosperms 
and  in  such  Pteri- 
dophytesaslsoetes.  A 

As  in  the  Gymno-  Mil*- 

sperms,  the  macro- 
spore  remains  per- 
manently within 
the  ovule.  The 
gametophy  te  is  usu- 
ally extremely  re- 
duced, showing  in 
the  typical  forms 
a  very  constant 
structure. 

The  single  nu- 
cleus of  the  ma- 
crospore  divides, 
and  one  of  the  two 
resulting  nuclei 
moves  to  each  end 
of  the  spore-cavity 
or  embryo-sac. 
Here  each  nucleus 
divides  twice,  so 
that  there  result 
four  nuclei  at  each 
end  of  the  sac. 
Three  of  them  re- 


FIG.  44. — A,  diagram  showing  the  arrange- 
ment of  parts  in  a  typical  angiospermous 
flower ;  c.a,  the  calyx  made  up  of  individual 
leaves,  sepals ;  co,  the  corolla,  composed  of 
petals;  an,  the  andrcecium,  composed  of 
the  stamens;  gy,  the  gynoecium,  made  up 
of  the  carpels  (here  hut  a  single  carpel). 
The  upper  part,  anther,  an,  of  each  stamen 
has  usually  four  microsporangia,  or  pollen- 
sacs;  the  macrosporangia,  or  ovules,  are 
contained  in  the  ovary,  formed  hy  the  hase 
of  the  carpel,  or  united  carpels  ;  p,  a  pol- 
len-spore germinating  upon  the  stigma,  and 
sending  its  tube,  pt,  down  through  the  cen- 
tral part  of  the  gynoecium  or  pistil  ;  B,  dia- 
gram showing  the  structures  within  the 
embryo-sac  at  the  time  of  fertilization. 
The  three  cells  at  the  upper  end  form  the 
"egg-apparatus,"  consisting  of  the  two 
synergids,  sy,  and  the  egg-cell,  o.  At  the 
lower  end  are  the  three  "  antipodal-cells," 
ant,  and  in  the  centre  are  the  two  "  polar 
nuclei,"  pn,  which  afterward  unite  into  a 


single  one,  the  "endosperm-nucleus";  C, 
young  pollen-spore  of  Naias,  showing  the 
antheridial  cell,  x ;  D,  a  germinating  pollen- 
spore  of  the  sweet-pea;  pt,  pollen-tube;  x, 
sperm-nucleus. 


main  at  the  ends, 

while  the  fourth  one  from  each  end  moves  toward  the 

centre  of  the  embryo-sac  where  these  two  "polar  nuclei  " 


180  EVOLUTION   OF   PLANTS 

i 

(Fig.  44,  B,  pri)  coalesce.  About  the  three  nuclei  at 
the  upper  or  micropylar  end  of  the  embryo-sac  there  is 
formed  an  aggregation  of  protoplasm  resulting  in  three 
naked  cells,  which  constitute  the  so-called  "  egg-appa- 
ratus." The  three  nuclei  at  the  other  end  become  also 
surrounded  by  protoplasm,  which  usually  is  bounded 
by  a  definite  cell-wall.  These  three  cells  are  called  the 
antipodal  cells  (Fig.  44,  B,  ant). 

In  some  of  the  simpler  Monocotyledons,  e.g.  the 
grasses,  the  number  of  antipodal  cells  is  more  numer- 
ous, and  a  few  cases  are  known  where  there  seems  to 
be  a  multiplication  of  the  other  cells  within  the  embryo- 
sac  ;  but  these  are  not  yet  sufficiently  understood  to 
throw  much  light  upon  the  homologies  existing  between 
the  female  gametophyte  of  the  Angiosperms  and  that 
of  the  Gymnosperms  and  heterosporous  Pteridophytes. 
Whether  the  egg-apparatus  is  to  be  considered  as  a 
single  archegonium,  or  whether  each  of  its  cells  is  to 
be  so  regarded,  cannot  be  positively  decided  at  present. 
The  other  cells,  i.e.  the  antipodals  and  the  endosperm 
cells  formed  later,  represent  probably  the  vegetative 
part  of  the  gametophyte. 

The  microsporangia  are  much  less  modified,  and  the 
development  of  the  microspores  (pollen)  corresponds 
exactly  with  that  of  the  Gymnosperms  and  the  Arche- 
goniates  from  the  mosses  up,  even  to  the  final  division 
of  the  mother-cell  into  four  spores.  The  ripe  pollen- 
spore  shows  exactly  the  same  structure  as  the  spores 
of  the  Archegoniates.  On  germination  two  cells  are 
formed,  a  large  vegetative  one  and  a  small  antheridial 
cell  (Fig.  44,  C).  The  former,  when  the  spore  falls 
upon  the  pistil,  develops  into  the  pollen-tube,  and  the 


ANGIOSPERMJE  181 

antheridial  cell  passes  into  it.  Either  before  this 
happens,  or  later,  the  antheridial  cell  divides  into  two 
sperm-cells,  but  no  motile  spermatozoids  are  formed. 

The  upper  part  of  the  pistil,  the  stigma,  is  usually 
provided  with  papillae  which  hold  the  pollen,  and  the 
surface  is  often  adhesive  owing  to  a  peculiar  secretion 
which  at  the  same  time  probably  serves  to  induce  the 
germination  of  the  pollen.  In  some  cases  there  is  an 
open  tube  through  the  pistil,  through  which  the  pollen- 
tubes  grow,  but  more  commonly  the  central  part  of  the 
style  is  occupied  by  a  peculiar  conducting  tissue  serv- 
ing to  nourish  the  growing  pollen-tube,  which  grows 
through  it  much  as  a  fungus  hypha  grows  through  the 
tissues  of  its  host.  On  reaching  the  ovary,  the  pollen- 
tube  grows  along  the  placenta  or  tissues  from  which 
the  ovules  spring,  until  finally  it  reaches  the  micropyle, 
or  opening  of  the  ovule,  which  it  then  enters,  and  pen- 
etrates through  the  overlying  tissues  of  the  apex,  to 
the  egg-apparatus.  The  expulsion  of  the  generative 
cell  is  effected  much  as  in  the  Conifers,  and  one  of  the 
sperm-nuclei  makes  its  way  into  the  egg-cell,  the  lowest 
of  the  three  cells  of  the  egg-apparatus  (Fig.  44,  B,  o), 
the  two  others,  the  synergidee  (s«/),  probably  assisting 
in  the  transference  of  the  male  nucleus  from  the  pollen- 
tube  to  the  egg. 

The  effect  of  pollination  is  usually  marked  by  a  rapid 
growth  of  the  ovary,  as  well  as  the  development  of  the 
ovules  into  seeds.  The  development  of  the  latter  is 
quite  similar  to  that  in  the  Gymnosperms,  but  the 
further  changes  in  the  carpels,  to  form  the  "fruit,"  is 
peculiar  to  Angiosperms.  We  cannot  here  go  into 
details  as  to  the  great  variety  shown  in  the  fruits  of 


182  EVOLUTION   OF    PLANTS 

Angiosperms,  but  shall  refer  to  this  later.  The  effect 
of  fertilization  may  extend  beyond  the  fruit  itself  and 
involve  the  calyx,  as  in  the  apple  or  pear,  or  even  the 
summit  of  the  floral  axis,  as  in  the  strawberry  or  fig. 

The  embryo-sporophyte  in  different  Angiosperms 
shows  a  very  different  degree  of  development  at  the 
time  the  seed  ripens.  Sometimes,  especially  in  para- 
sitic plants,  it  consists  merely  of  a  small  mass  of  cells 
without  any  external  differentiation.  On  the  other 
hand,  as  in  the  pea  family,  it  finally  occupies  the  whole 
cavity  of  the  seed,  and  all  the  parts,  stem,  root,  and 
cotyledons,  and  the  terminal  bud,  are  perfectly  formed. 

The  remarkable  complexity  shown  by  the  fully  devel- 
oped sporophyte  of  the  Angiosperms  offers  a  marked 
contrast  to  the  extremely  reduced  gametophyte,  and  it 
is  in  this  group  that  the  development  of  the  sporophyte 
reaches  its  most  complete  expression.  From  jninute 
uiidifferentiated  aquatics  like  the  little  duckweed 
(Lemna)  (Fig.  45,  D),  every  grade  of  development  is 
encountered,  up  to  trees  rivalling  the  giants  among  the 
Conifers  in  point  of  size,  and  far  surpassing  them  in  the 
perfection  of  their  parts,  especially  the  flowers. 

In  marked  contrast  to  the  Gymnosperms,  which  are 
restricted  in  their  range,  some  forms  of  Angiosperms  oc- 
cur under  all  conditions.  Some  are  aquatics,  even  grow- 
ing in  the  ocean,  while  others  are  inhabitants,  of  almost 
absolute  deserts.  Some  are  stately  trees,  while  others 
are  minute,  almost  microscopic,  herbs  living  but  a  few 
weeks.  Especially  in  the  tropics,  where  the  struggle 
for  existence  is  keen,  do  we  find  Angiosperms  taking 
advantage  of  every  opportunity  offered,  —  some  lift 
themselves  by  tendrils  or  by  twining  their  stems  about 


ANGIOSPERM^E  183 

other  plants,  until  they  reach  the  light,  while  myriads 
of  parasites  and  air-plants  cover  the  trunks  and  branches 
of  the  trees,  all  striving  to  hold  their  own  in  the  fierce 
competition.  The  variety  shown  in  the  flowers  and 
fruits  of  these  plants,  as  well  as  in  the  leaves  and  stems, 
is  almost  infinite,  and  in  these  respects  the  Angiosperms 
stand  far  above  all  other  plants.  In  spite  of  this 
extraordinary  variety,  the  essential  structure  of  the 
flowers  and  seeds  of  the  Angiosperms  is  remarkably 
uniform,  and  with  little  question  they  constitute  a 
perfectly  homogeneous  class. 

The  Angiosperms  fall  naturally  into  two  subclasses, — 
Monocotyledons  and  Dicotyledons.  These  show  many 
points  of  similarity  in  their  structure,  but  the  differ- 
ences are  sufficient  to  make  somewhat  doubtful  the 
exact  relationship  of  the  two. 

THE  MONOCOTYLEDONS 

The  Monocotyledons  are  usually  simpler  than  the 
Dicotyledons,  both  as  regards  their  tissues  and  their 
flowers,  although  among  them  are  certain  groups,  like 
the  orchids,  which  are  among  the  most  specialized  of 
all  Angiosperms.  As  a  rule  they  have  narrow  leaves 
with  unbranched  veins,  and  the  vascular  bundles  never 
exhibit  secondary  thickening.  The  roots  therefore 
never  become  very  thick,  and  a  tap-root  is  never  devel- 
oped. While  it  is  true  that  the  parts  of  the  flower  are 
usually  arranged  in  whorls  of  three,  there  are  so  many 
exceptions  to  the  rule  that  it  cannot  be  used  as  a  satis- 
factory diagnostic  character  of  the  group  as  a  whole. 

The  embryo  of  the  Monocotyledons  is  characterized 


184  EVOLUTION  OF   PLANTS 

by  the  presence  of  a  single  primary  leaf,  or  cotyledon, 
which  usually  arises  from  the  apex  of  the  embryo,  the 
stem-apex  of  the  young  sporophyte  in  most  cases  being 
formed  laterally  (Fig.  45,  G).  Of  the  Pteridophytes, 
Isoetes  shows  the  nearest  approach  to  the  conditions 
found  in  typical  Monocotyledons.  There  is  much  uncer- 
tainty at  present  as  to  which  of  the  Monocotyledons  are 
to  be  considered  as  the  most  primitive,  and  their  relation 
to  the  other  Spermatophytes  is  also  a  question  about 
which  there  is  much  disagreement.  These  points  can  be 
settled  only  after  much  more  is  known  than  at  present 
about  the  development  of  the  flower  and  embryo  in  the 
simpler  types  of  the  group.  The  principal  disputed 
point  at  present  is  whether  the  forms  with  the  simplest 
flowers  are  really  the  most  primitive,  or  whether  this 
simplicity  is  a  reduction  from  a  more  specialized  type. 

The  Monocotyledons  which  possess  the  simplest 
flowers  are  aquatics,  the  simplest  of  all  being  probably 
the  genus  Naias  (Fig.  43).  In  this  genus,  which  is  com- 
posed of  completely  submerged  aquatics,  the  flowers 
are  reduced  to  a  single  carpel  or  stamen,  the  latter  usu- 
ally showing  but  a  single  pollen-sac  or  sporangium, 
produced  directly  from  the  transformed  apex  of  a 
shoot ;  the  ovule  originates  in  precisely  the  same  way. 
Both  kinds  of  sporangia  are  remarkably  alike  in  their 
early  stages,  and  the  origin  of  the  sporogenous  tissue 
is  the  same  in  both,  and  suggests  that  of  many  Pterido- 
phytes. Whether  or  not  this  simple  structure  of  the 
flower  in  Naias  is  the  result  of  reduction  from  a  more 
specialized  type,  it  is  certainly  more  like  the  sporangia 
of  the  Pteridophytes  than  is  that  of  any  other  Angio- 
sperm.  A  similar  type  of  flower,  but  somewhat  more 


ANGIOSPEKM^: 


185 


complicated,  is  found  in  a  number  of  aquatic  forms 
allied  to  Naias,  and  also  occurs  in  some  of  the  terrestrial 
types  among  the  aroids.  In  the  latter  (Fig.  45,  A-C), 
while  the  individual  flowers  are  often  of  the  simplest 


FIG.  45  (Lower  Monocotyledons).  —  A,  female  inflorescence  of  the  Indian 
turnip  (Arisfema),  the  enveloping  bract  cut  away  at  the  base  to  show 
the  inconspicuous  flowers,  fl ;  B,  a  single  flower  cut  longitudinally  to 
show  the  ovules,  o  ;  st,  the  papillate  stigma ;  C,  a  group  of  male  flowers, 
each  consisting  of  four  stamens  ;  D,  two  plants  of  duckweed  (Lemna), 
a  minute  floating  aroid ;  //,  the  inflorescence  consisting  of  two  male  and 
one  female  flower  ;  E,  the  female  flower  cut  longitudinally ;  F,  the  male 
flower,  consisting  of  a  single  stamen  ;  G,  longitudinal  section  of  the  em- 
bryo of  Naias,  showing  the  characters  of  the  typical  monocotyledonous 
embryo;  the  cotyledon  is  terminal,  and  the  stem-apex,  st,  of  lateral 
origin;  r,  the  root;  sits,  suspensor;  H,  male  flower  of  arrow-head 
•  (Sagittaria),  consisting  of  a  group  of  stamens  surrounded  by  three 
white  petals,  p,  and  three  sepals,  s  ;  I,  section  through  the  head  of 
separate  carpels,  car,  from  the  female  flower  ;  J,  inflorescence  (spike- 
let)  of  a  grass  (Dactylis)  ;  the  lowest  flower  has  the  three  stamens,  and 
the  two  feathery  stigmas  protruding  ;  K,  a  separate  flower  of  Dactylis, 
consisting  of  a  single  carpel  and  three  stamens  in  the  axil  of  the  bract, 
p ;  at  the  base  of  the  carpel  are  the  two  small  bracts  (lodicules),  I. 

description,  they  are  usually  aggregated  to  form  a  com- 
pact, elongated  inflorescence,  the  spadix,  which  may 
reach  a  large  size  and  be  very  conspicuous,  especially 
when,  as  often  happens,  it  is  surrounded  by  a  showy 
bract,  as  in  the  common  "  calla  lily  "  or  some  species  of 


186  EVOLUTION   OF   PLANTS 

Anthurium.  This  bright-colored  "spathe"  serves  here 
the  purpose  of  the  showy,  corolla  of  the  higher  forms. 

Somewhat  higher  in  the  scale  are  found  plants  whose 
flowers  are  made  up  of  numerous  but  separate  sporo- 
phylls.  These  may  have  carpels  and  stamens  together 
in  the  same  flower,  or  they  may  be  separated,  as  in 
the  common  arrow-head  (Sagittaria),  (Fig.  45,  H,  J). 
In  these  there  are  also  found  the  accessory  leaves, 
sepals  (s)  and  petals  (j?),  the  latter  often  large  and 
showy.  These  forms  show  certain  analogies,  both  in 
the  structure  of  the  flowers  and  the  tissues,  with  some 
of  the  lower  Dicotyledons,  especially  the  buttercup 
family  (Ranunculacese),  and  it  has  been  suggested  that 
the  latter  may  have  been  derived  from  Monocotyledons 
of  this  type. 

Of  the  simplest  of  the  Monocotyledons,  the  Naiad- 
acese,  or  pond-weeds,  have  been  referred  to.  Other 
groups  which  are  considered  to  be  very  priniitive  are 
the  Cat-tail  rushes  (Typhacese),  the  Bur-reeds  (Sparga- 
niaceae),  the  Screw-pines  (Pandanacere),  as  well  as 
several  other  less-known  groups. 

The  Aroids  (Aracece),  of  which  the  common  calla  lily 
is  perhaps  the  best-known  representative,  show  many 
evidences  of  being  a  primitive  group,  especially  in  the 
simplicity  of  the  flowers,  although  there  is  considerable 
variety  among  them  in  this  respect.  They  are  for 
the  most  part  tropical,  although  a  few  genera,  Arum, 
Arisrcma,  Symplocarpus,  and  others,  are  inhabitants  of 
the  temperate  regions.  Some  of  the  tropical  aroids  are 
plants  of  considerable  size,  the  largest  being  climbers, 
whose  long  stems  may  reach  to  the  top  of  lofty  trees. 
These  climbing  aroids  are  among  the  most  striking  of 


ANGIOSPERM^E  187 

tropical  growths,  especially  in  the  American  tropics, 
where  some  species  of  Philodendron  and  Monstera  are 
among  the  most  conspicuous  plants  met  with.  The 
smallest  and  simplest  of  the  family  are  the  duckweeds 
(Lemna),  minute  floating  plants,  the  smallest  of  all 
flowering  plants  (Fig.  45,  D).  These  are  usually  con- 
sidered to  be  degenerate  relations  of  the  more  specialized 
aroids.  Some  of  the  latter  possess  true  compound  leaves, 
which  are  almost  unknown  elsewhere  among  the  Mono- 
cotyledons, and  in  this  respect  they  resemble  the  ferns 
and  many  Dicotyledons. 

Probably  remotely  connected  with  the  aroids  are  the 
Palms,  a  large  order  mainly  restricted  to  the  tropics, 
and  one  of  the  most  striking  types  of  the  vegetable 
kingdom.  A  few  genera,  like  the  palmettoes  of  the 
Gulf  States  and  the  fan-palms  of  southern  California, 
extend  beyond  the  tropics,  but  it  is  in  the  hot,  moist 
regions  of  the  tropics  that  they  reach  their  most  perfect 
development.  Most  of  the  palms,  as  is  well  known,  are 
unbranched  trees,  with  a  crown  of  gigantic  leaves,  either 
pinnate  or  fan-shaped.  The  apparently  compound  leaves 
of  palms  are  caused  by  the  tearing  into  strips  of  an  origi- 
nally simple  plaited  leaf,  such  as  occurs  permanently  in 
a  very  few  species,  and  is  always  found  in  the  seedling. 
The  palms  have  the  parts  of  the  flower  in  threes,  as  in 
the  higher  Monocotyledons,  and  they  may  be  either 
perfect  or  diclinous,  i.e.  bearing  only  carpels  or  stamens. 
In  the  latter  case  both  sorts  of  flowers  may  be  upon  the 
same  plant,  or  upon  different  individuals  as  in  the 
common  date-palm.  Just  what  relation  the  palms  bear 
to  the  aroids  is  doubtful,  but  there  is  a  peculiar  group 
of  plants,  the  Cyclantherse,  natives  of  the  American 


188  EVOLUTION   OF  PLANTS 

tropics,  which  seem  to  be  to  a  certain  extent  interme- 
diate between  the  two. 

Another  very  sharply  defined  order  of  Monocotyle- 
dons is  the  Graminese,  or  Grasses.  These  are  cosmo- 
politan in  their  distribution,  and  in  the  temperate  regions 
they  form  one  of  the  most  important  elements  of  the 
vegetation,  especially  over  open,  exposed  areas.  Eco- 
nomically they  are  the  most  important  of  all  plants,  as 
they  include  all  the  cereals,  as  well  as  sugar-cane  and 
bamboo,  and  are  the  most  important  food  plants  for 
herbivorous  animals.  The  number  of  grasses  exceeds 
that  of  any  other  group  of  Monocotyledons  except  the 
orchids.  They  are,  however,  very  uniform  in  the  struct- 
ure of  the  stem  and  leaves  as  well  as  of  the  flowers. 
The  stems  are  jointed  and  usually  hollow,  sometimes 
of  gigantic  size,  30-40  metres  in  some  of  the  bamboos. 
The  narrow,  two-ranked  leaves,  with  their  sheathing 
bases,  and  the  chaffy  scale.s  about  the  simple  "flowers, 
are  constant  characters  of  this  very  natural  family. 

Certain  peculiarities  of  the  ovule  and  the  gameto- 
phyte  indicate  that  the  grasses  belong  near  the  bottom 
of  the  series  of  Monocotyledons,  but  their  great  num- 
bers and  wide  distribution  show  that  they  have  become 
sufficiently  modified  to  adapt  them  very  perfectly  to 
existing  conditions. 

Owing  to  the  absence  of  any  forms  intermediate  be- 
tween the  grasses  and  the  other  Monocotyledons,  their 
exact  position  in  the  system  is  still  very  uncertain. 

Closely  resembling  the  grasses  externally,  but  differing 
from  them  in  many  important  particulars,  are  the  Sedges 
(Cyperacese),  which  are  usually  associated  with  the 
grasses  in  a  single  order,  Glumaceae.  It  seems  proba- 


ANGIOSPERM^E 


189 


ble,  however,  that  the  two  families  are  not  closely  re- 
lated, and  the  sedges  are  probably  more  nearly  related 
to  some  other  group  of  Monocotyledons,  possibly  the 
Rushes  (Juncacese)  than  to  the  grasses.  Whether  the 
simple  type  of  flower  found  in  the  grasses  and  sedges 


nx: 


FIG.  46  (Monocotyledons  —  Liliiflorse).  — A,  a  plant  of  yellow  adder-tongue 
(Erythronium ) ,  a  typical  liliaceous  plant ;  B,  the  underground  thickened 
stem  or  bulb,  with  the  simple  roots  growing  from  it ;  C,  the  pistil,  com- 
posed of  three  united  carpels;  D,  diagram  showing  the  arrangement  of 
the  parts  of  the  flower ;  E,  flower  of  Narcissus,  differing  from  the  true 
lilies  in  having  an  "inferior"  ovary,  o:  F,  flower  of  an  Iris,  a  highly 
specialized  flower  adapted  to  insect  pollination ;  G,  cross-section  of  the 
stem  of  Iris,  showing  the  arrangement  of  the  tissues  in  a  typical  mono- 
cotyledonous  stem  ;  vb,  the  vascular  bundles  ;  H,  a  flower  of  the  pick- 
erel-weed (Pontederia)  ;  the  flower  is  strongly  "  zygomorphic,"  i.e. 
bilaterally  symmetrical,  and  the  stamens  are  in  two  sets. 

is  primitive,  or  whether  it  is  the  result  of  reduction  from 
a  more  complex  one,  must  remain  for  the  present  unde- 
cided. 

All  of  the  higher  Monocotyledons  are  distinguished 
by  much  more  specialized  flowers  than  those  found  in 
the  forms  just  considered.  This  specialization  maoi- 


190  EVOLUTION   OF  PLANTS 

fests  itself  first  in  the  development  of  a  colored  peri- 
anth or  floral  envelope,  hence  they  are  known  as  the 
"petaloideous "  Monocotyledons.  Most  of  these  have 
the  carpels  more  or  less  perfectly  grown  together  into 
a  compound  pistil. 

In  the  great  majority  of  the  petaloideous  forms  the 
parts  of  the  flower  are  in  whorls  of  three,  the  typical 
arrangement  being  shown  in  the  accompanying  dia- 
gram (Fig.  46,  D).  The  two  sets  of  leaves  constituting 
the  perianth  are  usually  alike  in  color  and  texture,  but 
occasionally,  e.g.  Trillium,  the  outer  leaves  are  green, 
and  form  a  calyx  like  that  found  in  most  Dicotyledons. 
The  six  stamens  are  in  two  alternating  whorls,  and  the 
three  carpels  completely  coherent. 

Probably  the  lowest  of  the  petaloideous  series  with 
coherent  carpels  are  the  lilies,  with  their  regular  flowers 
showing  perfect  radial  symmetry.  Here  are  found 
many  of  the  most  magnificent  of  all  flowers,  and  the 
brilliant  colors  and  fragrance  of  many  of  them  show 
their  adaptation  to  insect  aid  in  their  pollination. 

Starting  from  the  type  exhibited  by  the  simpler  mem- 
bers of  the  lily  family,  it  is  easy  to  see  how  specializa- 
tion has  progressed  in  different  directions.  This  is  first 
seen  in  the  coherence  of  the  leaves  of  the  perianth,  so 
that  the  flower  becomes  tubular,  as  in  the  hyacinth  or 
tuberose.  This  is  sometimes  accompanied  by  a  slight 
inequality  in  the  size  of  the  perianth  lobes,  especially 
if  the  flower  is  nodding,  and  in  such  cases  the  stamens 
and  pistil  are  declined  so  that  the  flower  is  more  or  less 
markedly  two-lipped  (Fig.  46,  H).  Carried  further,  the 
cohesion  of  the  perianth  extends  to  the  pistil,  and  the 
result  is  a  tubular  flower  with  a  so-called  "inferior" 


ANGIOSPEKM2E  191 

ovary  (Fig.  46,  E).  In  such  flowers  the  base  of  the 
perianth  is  completely  adherent  to  the  ovary,  so  that 
the  outer  part  of  the  latter  is  completely  fused  with  the 
base  of  the  perianth-tube,  and  the  perianth  appears  to 
be  attached  to  the  top  of  the  ovary.  Familiar  examples 
of  this  are  seen  in  the  various  species  of  Narcissus  (Fig. 
46,  E),  Amaryllis,  and  other  members  of  the  Amaryllis 
family. 

Much  more  profound  modifications  of  the  lily  type 
are  met  with  in  the  Iris  family.  Here  the  cohesion  of 
the  parts  of  the  flower  is  accompanied  by  a  suppression 
of  one  set  of  stamens,  and  in  some  of  them  the  flowers 
are  strongly  zygomorphic,  i.e.  bilaterally  symmetrical, 
as  in  Gladiolus.  The  genus  Iris  (Fig.  46,  F)  is  per- 
haps the  most  specialized  of  the  family,  the  peculiar 
arrangement  of  the  floral  parts,  especially  the  stamens 
and  pistil,  being  such  as  to  render  insect  aid  abso- 
lutely necessary  in  order  that  pollination  may  be 
effected. 

Some  of  the  lily  family  reach  the  dimensions  of  trees, 
showing  a  secondary  increase  in  the  thickness  of  the 
sterns,  a  rare  occurrence  among  the  Monocotyledons. 
This  is  brought  about,  however,  not  by  the  contin- 
ued growth  of  the  primary  vascular  bundles  as  in  the 
Gymnosperms,  but  by  a  zone  of  growing  tissue  in  the 
ground-tissue,  within  which  new  vascular  bundles  of 
limited  growth  develop,  so  that  a  section  of  the  stem 
of  one  of  these  arborescent  Liliacese  does  not  show 
definite  growth-rings,  but  appears  as  a  mass  of  nearly 
uniform  parenchyma,  in  which  are  imbedded  the 
numerous  isolated  vascular  bundles.  The  Yuccas  of 
the  southern  United  States,  and  the  Dracaenas  and 


192  EVOLUTION  OF  PLANTS 

allied  forms  of  the  Old  World,  are  the  best-known 
examples  of  these  arborescent  lilies. 

Various  other  adaptations  of  the  vegetative  parts  are 
shown  by  many  Liliiflorse.  Most  of  them  are  herba- 
ceous forms,  which  develop  underground  stems  capable 
of  resisting  extremes  of  both  cold  and  dryness.  These 
are  either  bulbs,  tubers,  or  similar  shortened  and  thick- 
ened subterranean  stems  in  whose  cells,  or  those  of 
thick  scale-leaves  growing  from  them,  are  stored  up 
starch  and  other  food-materials.  These  bulbs  and  tubers 
can  endure  complete  drying  up  without  injury,  and 
remain  dormant  during  the  long  periods  of  cold  or 
extreme  drought  to  which  the  plant  may  be  subjected, 
and  start  into  growth  very  quickly  on  the  advent  of 
favorable  growing  conditions,  drawing  upon  the  stored 
reserve  food  until  the  new  leaves  and  roots  are  de- 
veloped. 

These  bulbous  liliaceous  plants  are  especially  de- 
veloped in  those  countries  which  have  a  marked  wet 
and  dry  season.  California  and  the  Cape  district  of 
Africa  illustrate  this,  both  of  these  regions  being  nota- 
bly rich  in  liliaceous  plants,  many  of  them  having 
flowers  of  great  beauty. 

The  tendency  to  form  zygomorphic  flowers,  found 
occasionally  in  the  Liliiflorse,  becomes  the  rule  in  the 
two  most  specialized  orders  of  the  Monocotyledons,  the 
Scitaminese,  and  the  Gynandrse.  The  former  comprises 
the  Ginger  and  Banana  families,  as  well  as  the  familiar 
Cannas  of  the  gardens.  The  Gynandraa  include  two 
families,  of  which  the  Orchid  family  is  by  far  the  more 
important,  and  includes  a  very  large  majority  of  the 
forms.  Both  Scitaminese  and  Gynandrse  are  character- 


ANGIOSPERM^  193 

ized  by  an  inferior  ovary  and  strongly  zygomorphic 
flowers,  with  a  reduction  in  the  number  of  stamens 
often  to  a  single  one.  These  two  orders  represent  un- 
doubtedly the  highest  degree  of  specialization  among 
the  Monocotyledons. 

THE   SCITAMTNE^E 

The  Scitamineee  are,  with  very  few  exceptions,  tropical 
plants  of  very  striking  and  characteristic  appearance. 
They  are  mostly  plants  of  large  size  with  very  large 
leaves  and  often  showy  flowers.  Many  of  them  are 
cultivated  for  the  beauty  of  their  foliage  and  flowers, 
like  the  species  and  varieties  of  Canna  (Fig.  47,  A), 
while  others,  like  the  ginger,  and  especially  the  banana 
and  plantain,  are  important  food  plants.  They  usually 
have  a  thick  underground  rhizome  from  which  are  sent 
up  the  strong  shoots,  whose  large  leaves  when  young 
are  usually  rolled  up  like  a  cornucopia.  Each  shoot  in 
most  of  them  terminates  in  a  large  inflorescence,  and 
after  the  fruit  is  ripe  the  shoot  dies.  Occasionally  the 
growth  of  the  stem  is  not  checked  by  the  formation  of 
flowers,  and  it  may  assume  almost  tree-like  proportions, 
as  in  the  curious  "  traveller's  tree,"  Ravenala.  The 
flowers  of  some  genera,  like  Canna,  are  themselves  very 
showy,  but  quite  as  often  the  showy  inflorescence  owes 
its  attractiveness  to  the  bright-colored  bracts  in  whose 
axils  the  inconspicuous  flowers  are  borne.  This  is  well 
illustrated  by  the  gaudy  yellow  or  scarlet  bracts  of  Heli- 
conia  and  the  pink  or  crimson  ones  of  many  species  of 
Zingiber. 

Another  peculiar  order  of  Monocotyledons  confined  to 


194 


EVOLUTION   OF   PLANTS 


the  New  World,  is  the  Bromeliacese.  These  are  char- 
acterized by  modifications  of  the  vegetative  parts  rather 
than  by  the  flowers,  which  are  rather  simple  in  structure. 
Most  of  the  order  are  epiphytes,  and  they  form  one  of 
the  most  striking  features  of  the  tropical  American  flora. 
The  best  known  of  these  are  the  so-called  "  Spanish 


FIG.  47  (Monocotyledons,  Scitaminene,  Orchidaceae) .  —  A,  flower  of  Canna : 
the  flower  is  strongly  zygomorphic,  with  inferior  ovary,  o,  and  the 
stamens  reduced  to  a  single  one;  B,  the  single  stamen"  an,  and  the 
upper  part  of  the  pistil,  st,  of  A;  C,  flower  of  an  orchid  (Arethusa), 
showing  the  marked  zygomorphy,  inferior  ovary,  o,  and  the  "lip,"  I; 
D,  a  section  through  the  "column,"  or  coherent  stamen  and  pistil  of 
Arethusa,  showing  the  single  anther,  an,  and  the  stigma,  st:  the  rela- 
tive positions  of  the  anther  and  the  stigma  are  such  that  insect-pollina- 
tion is  absolutely  necessary ;  E,  flower  of  the  wild  yellow  lady's-slipper 
(Cypripedium  pubescens),  one  of  the  orchids;  I,  the  sac-shaped  lip; 
F,  the  column  of  the  lady's-slipper,  showing  one  of  the  two  fertile  sta- 
mens, an,  the  stigma,  st,  and  the  third,  sterile  stamen,  x. 

moss,"  of  the  southeastern  United  States  (Tillandsia 
usneoides),  and  the  cultivated  pineapple.  Several  spe- 
cies occur  in  Florida,  but  it  is  further  south  that  they 
reach  their  greatest  development.  In  the  West  Indies 
they  are  abundant  and  varied,  and  form  a  very  conspic- 
uous feature  of  the  vegetation,  covering  the  branches  of 


ANGIOSPERM^E  195 

the  trees  with  great  masses  of  their  spiky  leaves,  with 
here  and  there  clusters  of  showy  crimson  bracts,  or  in 
some  cases  gayly  colored  flowers.  The  broad,  overlap- 
ping leaf-bases,  and  the  scales  upon  them,  form  efficient 
reservoirs  both  for  water  and  the  accumulation  of  vege- 
table mould,  which  these  "air-plants"  need  for  their 
subsistence,  as  they  are  in  no  sense  parasites  upon  the 
trees  to  which  they  are  attached. 

Probably  to  be  regarded  as  the  most  specialized  of  all 
the  Monocotyledons  are  the  Orchids.  In  these  the 
flower  is  strongly  zygomorphic  (Fig.  47,  C,  E),  and 
usually  one  petal  is  decidedly  different  from  the  others 
and  forms  the  "lip"  (7).  In  much  the  greater  number 
of  them  the  stamens  are  reduced  to  a  single  one,  which 
is  coherent  with  the  upper  part  of  the  pistil  and  forms 
with  it  the  "column"  (Fig.  47,  D,  F),  but  sometimes 
two  stamens  are  present.  Usually  the  pollen-spores 
are  held  together  in  masses  (pollinia)  by  a  viscid  sub- 
stance, and  the  position  of  the  pollinia  is  such  that 
insect  aid  is  necessary  to  dislodge  them  and  transfer 
the  pollen  to  the  stigma.  We  find,  consequently,  among 
the  orchids  a  wonderful  variety  of  ingenious  devices  by 
which  cross-fertilization  is  effected.  Sometimes  the 
flower  is  adapted  to  pollination  by  a  single  species  of 
insect  upon  which  it  is  absolutely  dependent. 

In  spite  of  these  perfect  adaptations  for  cross-fertiliza- 
tion, the  orchids  seem  for  some  reason  to  be  less  per- 
fectly suited  to  their  environment  than  many  other 
plants.  They  seldom  occur  in  such  great  numbers 
together  as  to  make  much  of  an  impression  upon  the 
aspect  of  the  vegetation  as  a  whole,  although  individually 
they  are  often  among  the  showiest  of  flowers.  Compared 


196  EVOLUTION   OF   PLANTS 

with  the  Composite  among  the  Dicotyledons,  or  with  the 
grasses,  which  they  far  outnumber  in  species,  they  give 
the  impression  of  a  group  of  plants  in  a  formative  con- 
dition, which  has  not  yet  reached  a  stage  which  fits  them 
to  compete  successfully  with  their  hardier  rivals. 

Among  the  interesting  modifications  shown  by  the 
orchids  and  not  found  elsewhere  among  the  Monocoty- 
ledons (except  in  the  nearly  related  Burmanniaceae),  is 
the  adoption  of  the  saprophytic  habit  by  some  of  them. 
Such  forms,  e.g.  Corallorhiza,  are  characterized  by  a 
partial  or  complete  loss  of  chlorophyll,  with  a  corre- 
sponding reduction  of  the  leaves,  which  are  small  and 
scale-like. 

SUMMARY 

Considering  the  Monocotyledons  as  a  whole,  they  are 
much  less  numerous  than  the  Dicotyledons  as  well  as 
simpler  in  structure,  and  these  points  together  with  cer- 
tain structural  resemblances  between  them  and  the  ferns 
seem  to  indicate  that  they  are  the  more  primitive  of  the 
two  great  divisions  of  the  Angiosperms,  and  it  is  not 
improbable  that  they  have  originated  directly  from 
pteridophytic  ancestors,  or  possibly  through  forms 
related  to  the  Cycads. 

It  seems  likely  that  the  lowest  of  the  Monocotyledons 
are  the  simple  aquatic  forms  like  Naias,  where  the  flower 
consists  of  a  single  stamen  or  carpel,  and  from  these  the 
higher  types  with  hermaphrodite  flowers,  and  later  those 
with  a  showy  perianth,  have  been  derived.  It  is  true  that 
many  botanists  consider  the  extreme  simplicity  of  the 
flowers  of  the  aquatic  Monocotyledons  to  be  a  reduced 


ANGIOSPERM^E  197 

condition,  but  there  is  no  evidence  of  such  reduction 
shown  by  a  study  of  their  development,  and  they  can- 
not readily  be  referred  to  any  of  the  higher  types  of 
flowers. 

From  the  apocarpous  type,  ^.e.that  in  which  the  carpels 
are  all  distinct,  the  next  step  in  the  evolution  of  the 
flower  is  the  development  of  a  flower  like  that  of  the 
lilies,  with  the  carpels  united  to  form  a  compound  pistil, 
usually  composed  of  three  parts.  In  this  type,  which  is 
usually  considered  the  central  type  of  the  Monocotyle- 
dons, the  prevailing  number  of  the  different  organs  is 
three. 

From  the  lily  type  may  be  readily  derived  all  the 
higher  petaloideous  forms,  the  Iridacese,  the  Scitamineao 
and  the  Orchidacese.  In  these  there  is  a  cohesion  of  cer- 
tain parts  of  the  flower  and  usually  a  reduction  in  the 
number  of  stamens.  On  the  whole,  the  orchids  repre- 
sent the  most  highly  specialized  types. 

The  affinities  of  certain  other  groups  are  not  so  obvi- 
ous. The  grasses,  palms,  and  aroids  cannot  readily  be 
referred  to  the  same  series  as  the  lilies,  and  it  is  likely 
that  each  of  these  groups  has  been  derived  directly  from 
apocarpous  ancestors.  The  palms  and  aroids  show  cer- 
tain points  in  common,  while  the  latter  group  resembles 
in  certain  respects  the  aquatic  forms,  like  some  of  the 
pond-weeds  and  their  allies,  and  is  probably  related  to 
them. 

The  grasses  must  remain  for  the  present  very  much 
by  themselves.  Perhaps  a  thorough  study  of  their 
embryology  may  throw  some  light  upon  their  affinities, 
which  at  present,  it  must  be  admitted,  are  very  obscure. 


198 


EVOLUTION   OF  PLANTS 


Dicotyledons 


Orchids 
Orchidacese 


Banana,  ginger,  etc. 
(Scitamineae) 


Dicotyledons 


Aroids 
(Aracese) 


Iris,  etc. 
(Iridacese) 


Palms 
(Palmacese) 


Lilies 


s 

Bffi) 

,  etc. 
cere) 

Grasses 
(Grammefe) 


Apocarpre 


Screw-pines 

(Pandanaceae) 

Bur-reeds. 
(Sparganiacese) 


Pond-weeds 
(Naiadaceas,  etc.) 

Diagram  to  illustrate  the  relationships  of  the  principal  orders  of 
Monocotyledons. 


CHAPTER  XI 

DICOTYLEDONS 

THE  Dicotyledons,  the  second  great  division  of  the 
Angiosperms,  comprise  the  major  part  of  existing  plant 
forms,  and  it  is  among  these  that  the  vegetable  organism 
reaches  its  most  complete  expression.  Compared  with 
the  Monocotyledons  they  are  both  more  numerous  and 
more  varied.  With  the  exception  of  the  grasses  and  a 
few  aquatic  types,  the  Monocotyledons  are  seldom  abun- 
dant enough,  at  least  in  temperate  regions,  to  give  a 
prevailing  character  to  the  vegetation  of  any  district ; 
the  Dicotyledons,  on  the  other  hand,  are  often  gregari- 
ous and  better  able  to  hold  their  own  in  the  struggle 
for  existence.  All  the  forest  trees  of  temperate  regions, 
except  Conifers,  are  Dicotyledons,  and  except  for  the 
grasses,  hardly  any  of  the  aggressive  plants  we  call 
weeds  are  Monocotyledons,  and,  as  we  have  seen,  very 
few  types  of  the  Monocotyledons  attain  the  size  of  trees. 

The  most  constant  character  shown  by  the  Dicotyle- 
dons is  the  presence  of  two  cotyledons  or  primary 
leaves  in  the  embryo  (Fig.  48).  A  few  cases  where 
only  a  single  cotyledon  is  present  can  usually  be 
accounted  for  by  the  abortion  of  one  of  the  cotyledons, 
but  it  is  possible  that  there  may  be  forms  which  are 
intermediate  in  this  respect  between  the  two  great 

199 


200 


EVOLUTION   OF   PLANTS 


-cot 


groups  of   Angiosperms,   although  at  present  no  such 

forms  are  certainly  known. 

The  Dicotyledons  exhibit  great  variety  in  the  form  of 

the  stem  and  leaves,  and  this 
is  correlated  with  a  much 
more  perfect  development  of 
the  tissues  than  is  found 
elsewhere  in  the  vegetable 
kingdom.  This  is  shown 
especially  in  the  highly  de- 
veloped vascular  bundles, 
which  in  the  stems  of  the 
woody  forms  show  a  second- 
ary thickening  like  that  in 
the  coniferous  stem,  but  the 
tissues  of  the  bundle  are 
much  more  specialized  than 
in  the  latter.  From  the 
continued  growth  of  the 
cambium  or  active  tissue  in 
the  bundles  of  the  stem,  an- 
nual growth-rings  result,  and 
soon  the  greater  part  of  the 
stem  is  made  up  of  the  sec- 
ondary wood  derived  from 
the  activity  of  the  cambium. 
This  results  in  the  develop- 
ment of  the  massive  woody 
stems  characteristic  of  dico- 
tyledonous shrubs  and  trees. 

In  this  secondary  thickening  of  the  stems  and  roots,  the 

Dicotyledons  differ  from  the  Monocotyledons  and  ap- 


FIG.  48.  —  A,  a  seedling  of  the 
castor-bean  (Ricinus)  /showing 
the  difference  in  appearance 
between  the  two  cotyledons, 
cot,  and  the  second  leaves,  /; 
r,  the  main  or  tap-root,  a  contin- 
uation of  the  stem  ;  B,  cross- 
section  of  the  stem,  showing 
the  arrangement  of  the  tissues ; 
vb,  the  vascular  bundles;  C, 
section  of  the  seed  of  the 
shepherd's  purse  (Capsella),the 
embryo  occupying  the  whole 
seed-cavity;  cot,  cotyledons; 
st,  stem  :  D.  section  of  the  seed 
of  blood-root  (Sanguinaria), 
showing  the  small  embryo,  em. 


DICOTYLEDONS  201 

proach  the  Conifers,  where  a  similar  method  of  second- 
ary thickening  seems  to  have  been  developed  quite 
independently. 

While  the  leaves  of  the  Monocotyledons  are  usually 
linear,  with  parallel  venation,  those  of  the  Dicotyledons 
exhibit  great  variety  in  outline  and  venation.  They 
may  occasionally  have  simple  leaves  much  like  those 
of  the  typical  Monocotyledons,  but  much  oftener  the 
leaves  are  broadly  expanded,  with  a  clearly  defined 
petiole  or  stalk,  and  a  broad  lamina  with  reticulate 
venation.  The  base  of  the  petiole  is  often  provided 
with  small  leaf -like  appendages,  stipules.  The  ar- 
rangement of  the  veins  varies  with  the  shape  of  the 
lamina,  but  is  always  more  or  less  clearly  reticulated. 
There  may  be  only  one  main  vein  or  midrib,  or  there 
may  be  several  large  veins  of  nearly  equal  size  radiat- 
ing from  the  junction  of  the  lamina  and  petiole.  While 
the  margin  of  the  leaf  may  be  smooth  as  in  most  Mono- 
cotyledons, it  is  oftener  variously  indented  or  lobed, 
and  this  may  be  carried  so  far  as  to  result  in  a  complete 
division  of  the  lamina  into  separate  leaflets,  and  thus 
compound  leaves  like  those  of  the  ferns  arise.  The 
size  of  the  leaves  is  largely  dependent  upon  the  condi- 
tions of  growth,  and  in  plants  of  very  dry  regions,  or 
in  some  parasites  and  saprophytes,  the.  leaves  may  be 
entirely  wanting.  Where  leaves  are  entirely  absent 
their  place  may  be  taken  by  portions  of  the  stem,  whose 
outer  cells  develop  chlorophyll. 

The  variation  in  the  stem  is  quite  as  marked  as  that 
of  the  leaves.  The  stems  may  be  herbaceous  or  woody ; 
extremely  short,  even  bulbous,  like  many  Monocotyle- 
dons ;  or  they  may  be  enormously  lengthened,  slender 


202  EVOLUTION  OF  PLANTS 

twining,  or  creeping  ones;  or  again,  branches  may  be 
modified  into  thorns  or  tendrils.  Underground  por- 
tions of  the  stem  frequently  develop  stolons  or  tubers 
which  serve  to  propagate  the  plant.  These  are  but  a 
few  of  the  manifold  forms  which  the  dicotyledonous 
stems  may  assume. 

The  flowers  of  the  Dicotyledons  show  much  the  same 
general  structure  as  those  of  the  Monocotyledons,  and 
there  is  much  the  same  difference  between  the  highest 
and  the  lowest  types,  the  latter  hardly  surpassing  the 
simplest  ones  found  among  the  Monocotyledons.  While 
there  is  much  variation  in  the  number  of  parts  in  the 
flowers,  it  may  be  said  that  in  the  higher  types  the 
parts  —  at  least  sepals  and  petals,  and  frequently  the 
stamens  —  are  most  commonly  in  fives.  The  number 
of  carpels  is  usually  smaller. 

As  might  be  expected  from  the  great  diversity  shown 
in  the  flowers,  there  is  also  great  variety  in  the  char- 
acter of  the  fruit  and  seeds,  much  more  so  than  is  the 
case  among  Monocotyledons.  A  further  discussion  of 
this  point,  however,  will  be  left  for  a  later  chapter. 

The  carpels  and  stamens  of  the  typical  Dicotyledons 
resemble  closely  those  of  the  Monocotyledons.  The 
ovule  has  the  same  structure,  but  in  many  types  has 
but  a  single  integument,  this  being  especially  the  case 
in  the  highest  group,  the  Sympetalse.  The  macrospore 
(embryo-sac)  originates  in  the  same  way,  and  the  fully 
developed  gametophyte  shows  the  egg-apparatus  at  the 
upper  end  of  the  sac,  with  the  three  antipodals  at  the 
lower  end.  The  latter,  however,  may  in  exceptional 
cases  be  considerably  increased  in  number. 

The  development  of  the  stamen  and  the  pollen-sacs 


DICOTYLEDONS  203 

shows  nothing  peculiar.  The  pollen-spores  are  gener- 
ally of  the  same  tetrahedral  type  found  in  the  lowest 
Archegoniates,  and  we  see  that  even  in  these  highest  of 
all  plants  the  microspores  have  hardly  departed  from 
the  primitive  type  found  in  the  lowest  liverworts,  the 
division  of  the  spore  mother-cell  and  the  structure  of 
the  ripe  spores  being  identical  in  both.  The  germina- 
tion of  the  microspores  and  fertilization  are  as  in  Mono- 
cotyledons. 

The  development  of  the  embryo  follows  at  first  much 
the  same  course  as  in  Monocotyledons,  but  very  early 
there  is  in  most  cases  a  marked  difference  manifested. 
In  the  Monocotyledons,  as  a  rule,  the  apex  of  the 
embryo  becomes  transformed  into  the  single  cotyledon, 
the  stem-apex  being  formed  laterally ;  but  in  typical 
Dicotyledons  the  apex  of  the  embryo  forms  the  stem- 
apex,  while  the  two  opposite  cotyledons  are  developed 
secondarily  as  lateral  appendages  of  it.  It  may  be  stated, 
however,  that  Monocotyledons  are  known  in  which  the 
stem  is  derived  from  a  portion  of  the  apex  of  the  young 
embryo,  and  it  is  possible  that  a  similar  condition  may 
obtain  in  some  of  the  lower  Dicotyledons.  At  present 
our  knowledge  of  the  embryogeny  of  the  lower  mem- 
bers of  both  of  the  great  divisions  of  Angiosperms  is 
far  from  complete. 

The  degree  of  development  attained  by  the  embryo 
before  the  seed  ripens,  varies  a  good  deal  in  different 
Dicotyledons.  In  some  forms,  especially  saprophytes 
and  parasites  with  minute  seeds,  e.g.  the  Indian  pipe 
(Monotropa),  the  embryo  in  the  ripe  seed  is  completely 
un differentiated  and  consists  of  a  few  cells  only. 
Usually,  however,  it  is  well  developed,  and  the  primary 


204  EVOLUTION   OF   PLANTS 

organs,  stem,  root,  and  cotyledons,  are  readily  made  out. 
The  embryo  may  be  imbedded  in  the  endosperm  and 
not  occupy  the  whole  of  the  seed-cavity  (Fig.  48, 
D),  but  more  often,  perhaps,  the  endosperm  is  com- 
pletely absorbed  before  the  seed  ripens,  and  the  large 
embryo  fills  the  seed  completely,  as  we  see  in  all  legu- 
minous plants.  In  such  embryos  the  cotyledons  are 
very  large  and  thick,  and  their  cells  are  filled  with 
starch  and  other  food-substances  which  are  used  up  in 
the  early  stages  of  germination  (Fig.  48,  C). 

The  cotyledons  usually  differ  a  good  deal  in  shape 
from  the  later  leaves  (Fig.  48,  A),  which  gradually 
acquire  their  perfect  form.  The  cotyledons,  where  the 
embryo  fills  the  seed,  are,  as  we  have  seen,  thick  and 
fleshy,  with  obscure  veins ;  but  where  the  embryo  does 
not  fill  the  seed,  and  endosperm  is  present,  they  are 
usually  more  like  the  later  leaves,  being  thin  with 
prominent  veins,  as  in  the  morning-glory. 

None  of  the  Dicotyledons  occur  as  submersed  marine 
plants,  but  otherwise  they  are  found  in  nearly  every 
situation  where  plants  can  grow  at  all.  They  may  be 
completely  immersed  in  fresh  water,  e.g.  bladder-weed 
(Utricularia),  or  the  leaves  may  float  as  in  the  water- 
lilies,  while  many  of  them  are  inhabitants  of  swamps, 
where  they  are  more  or  less  completely  submerged. 
Many  of  them  live  in  the  sand  of  the  seashore,  while 
others  are  desert  plants.  The  various  forms  of  sage- 
brush and  cacti  of  our  own  Western  arid  regions  are 
excellent  types  of  these  "  xerophytic "  Dicotyledons. 
In  these  the  evaporating  surface  is  greatly  reduced  by 
the  minute  size  of  the  leaves,  and  loss  of  water  is  further 
retarded  by  excessive  thickening  of  the  outer  tissues  of 


DICOTYLEDONS  205 

the  stem  and  leaves.  These  thick  protective  tissues  also 
serve  to  shield  the  underlying  green  cells  from  the  too 
strong  rays  of  the  sun.  The  latter  result  is  also  brought 
about  in  many  desert  plants  by  the  development  of  a 
thick  covering  of  hairs  to  which  the  peculiar  gray  color 
of  many  of  these  is  due. 

Dicotyledons  are  among  the  last  plants  to  disappear 
upon  high  mountains,  and  some  of  them  have  been 
encountered  as  far  toward  the  poles  as  explorations  have 
extended. 

Among  the  Dicotyledons  are  found  the  most  extraor- 
dinary modifications  known  among  plants,  such  as  the 
remarkable  contrivances  developed  in  some  of  the  insec- 
tivorous plants  like  the  pitcher-plants  and  the  Venus's 
fly-trap.  It  is  among  these  also  that  the  most  perfect 
types  of  climbing  plants  are  found,  especially  those  with 
tendrils  of  various  patterns.  Parasites  and  saprophytes 
are  common  in  certain  families  of  Dicotyledons,  while 
among  the  Monocotyledons  they  are  rare.  The  mistle- 
toe and  dodder  are  familiar  examples  of  parasites,  while 
the  Indian  pipe  of  the  eastern  United  States,  and  its 
near  relative  the  curious  "  snow-plant "  (Sarcodes)  of 
the  Sierra  Nevada,  may  be  cited  as  typical  saprophytes. 
Everywhere,  except  in  the  sea,  where  any  vegetation 
exists  at  all,  we  encounter  the  ubiquitous  Dicotyledons. 

CLASSIFICATION  OF  DICOTYLEDONS:   CHORIPETAL,E 

The  Dicotyledons  may  be  divided  into  two  pretty  well- 
defined  great  divisions,  each  of  which  contains  numerous 
orders.  In  the  lower  series  (Choripetalse),  the  petals 
are  quite  separate,  and  this  may  be  true  of  other  parts 


206 


EVOLUTION  OF   PLANTS 


of  the  flower  as  well ;  but  in  a  larger  number  the  carpels 
are  more  or  less  completely  coherent,  and  the  sepals  are 
also  frequently  united  into  a  cup-shaped  or  tubular  calyx. 
The  lowest  of  the  Choripetalse  are  the  Arnentacese,  so 
named  from  usually  having  the  simple  flowers  in  elon- 
gated catkins  or  aments.  The  willows  (Fig.  49,  A-D), 
poplars,  and  various  nut-trees  are  familiar  examples  of 
this  group.  A  second  order  allied  to  these  includes  the 


FIG.  49  (Lower  Dicotyledons  —  Amentaceae,  Centrospermfe). — A,  male 
inflorescence  of  a  willow;  B,  an  individual  staminate  floWer  consist- 
ing of  two  stamens  surrounded  by  inconspicuous  bracts ;  C,  a  female 
inflorescence  of  a  willow,  each  flower  (D)  consisting  of  a  single  pistil 
made  up  of  two  coherent  carpels;  E,  flower  of  a  knot-grass  (Polygo- 
num)  :  the  perianth  consists  of  five  colored  sepals ;  F,  the  pistil  of  E, 
with  the  side  of  the  ovary  cut  away  to  show  the  single  ovule  borne  at 
the  apex  of  the  floral  axis  :  G,  section  of  the  flower  of  a  scarlet  catchrly 
(Silene)  ;  the  sepals  are  united  into  a  tube  enclosing  the  free  petals  and 
stamens  :  the  petals  are  showy,  and  the  flower  is  pollinated  by  insects  ; 
H,  diagram  of  the  flower  of  Silene ;  the  pistil  is  composed  of  three 
carpels ;  the  central  axial  placenta  bears  numerous  ovules. 

pepper  family,  a  tropical  group  which  superficially,  at 
least,  shows  a  curious  similarity  to  the  aroids,  and  may 
prove  to  comprise  connecting  forms  between  Mono- 
cotyledons and  Dicotyledons.  In  these  low  types  the 
flowers  are  often  diclinous,  i.e.  stamens  and  carpels  are 
in  separate  flowers  and  no  perianth  is  present,  or  the 
perianth  is  reduced  to  inconspicuous  scales.  It  is  gen- 
erally supposed  that  these  amentaceous  Dicotyledons 


DICOTYLEDONS 


207 


are  reduced  forms,  but  this  cannot  be  taken  for  granted, 
and  further  investigation  is  needed  before  definite  con- 
clusions can  be  reached  as  to  their  systematic  position. 

The  Amentacese  are  largely  inhabitants  of  the  cooler 
parts  of  the  world,  some  of  them,  like  the  willows  and 


-DC 


FIG.  50  (Polycarpicse j .  — A,  section  of  the  flower  of  a  buttercup  (Ranun- 
culus) ;  the  numerous  carpels  are  entirely  separate ;  B,  flower  of  wild 
columbine  (Aquilegia)  ;  the  petals,  p,  are  modified  into  tubular  necta- 
ries ;  C,  flower  of  a  larkspur  (Delphinium)  ;  the  flower  is  strongly 
zygomorphic,  and  the  two  upper  sepals  form  the  spur,  or  nectary ;  D, 
flower  of  the  tulip-tre'e  (Liriodendron),  one  of  the  Magnolia  family; 
the  flower  is  divided  lengthwise  to  show  the  numerous  stamens,  and 
the  separate  carpels  grouped  together  upon  the  elongated  central  recep- 
tacle; E,  flower  of  the  wild  lotus  (Nelumbo),  one  of  the  water-lily 
family ;  F,  young  fruit  of  the  lotus,  consisting  of  the  enlarged  conical 
receptacle,  with  the  separate  carpels,  car,  embedded  in  cavities  in  its 
upper  surface. 

birches,  being  among  the  most  northerly  of  all  trees  and 
shrubs. 

A  second  primitive  group  of  Choripetalse  is  the 
Polycarpicse  represented  by  the  buttercup  family  and 
its  allies.  Some  of  these  also  recall  one  group  of  the 
Monocotyledons,  the  Apocarpae,  e.g.  Alisma,  Sagitta- 


208  EVOLUTION   OF  PLANTS 

ria,  etc.,  in  the  character  of  the  flowers.  The  parts  of 
the  flower  are  all  separate,  and  in  the  lower  members  of 
the  group,  of  indefinite  number.  This  is  well  shown 
in  the  various  species  of  buttercups  (Ranunculus).  The 
Ranunculus  family  also  offers  some  interesting  examples 
of  specialization  within  a  group  which  nevertheless 
retains  a  very  primitive  type  in  the  arrangement  of  the 
floral  parts.  In  Anemone  (Fig.  55,  A)  and  in  Clematis, 
as  well  as  other  genera,  the  petals  are  quite  suppressed 
or  inconspicuous,  while  their  place  is  taken  by  the  large 
petaloid  sepals.  Some  other  genera,  like  the  columbines 
(Aquilegia),  larkspurs  (Delphinium),  and  moukshood 
(Aconitum),  have  the  parts  of  the  flower  extraor- 
dinarily modified  in  form,  and  yet  retain  the  primitive 
completely  separated  carpels  and  numerous  stamens 
(Fig.  50,  B,  C).  These  modifications  of  the  flower  are 
all  intimately  connected  with  insect-pollination,  and 
many  of  the  more  specialized  forms  like  Delphinium 
and  Aquilegia  are  probably  entirely  dependent  upon 
insects  or  humming-birds  for  pollination.  On  the  other 
hand,  some  species  of  Ranunculus  with  inconspicuous 
flowers  are  always  self-fertilized.  Other  Polycarpicae 
are  the  water-lilies  (Nymphseacese,  Fig.  50,  E),  magno- 
lias, and  several  other  less  familiar  families. 

Another  probably  primitive  group  of  the  Choripetalse 
is  the  order  known  as  the  CentrospermcC.  The  lowest 
members  of  the  series,  the  buckwheat  family  (Poly- 
gonacese,  Fig.  49,  E),  the  pig-weeds  (Chenopodiacese). 
etc.,  have  flowers  which  recall  the  peppers  and  some  of 
the  simple  Monocotyledons  in  having  the  single  ovule 
formed  directly  from  the  apex  of  the  floral  axis  (Fig. 
49,  F,  0).  The  higher  ones  have  numerous  ovules, 


DICOTYLEDONS  209 

which  also  arise  from  the  apex  of  the  axis,  which  here 
forms  the  placenta  (Fig.  49,  H).  The  lower  Centro- 
spermse  have  small,  inconspicuous  flowers  which  are 
principally  self-fertilized;  but  some  of  the  higher  ones, 
e.g.  the  pink  family,  often  exhibit  very  showy  flowers 
which  depend  upon  insect  aid.  In  these  more  special- 
ized types  the  calyx  is  usually  cup-shaped  or  tubular 
instead  of  being  composed  of  completely  separate  sepals 
(Fig.  49,  G).  Somewhat  higher  is  a  second  order 
(Cruciflorse),  including  the  Cruciferse  (mustard  family) 
and  poppies  (Papaveracese).  In  these  the  carpels  are 
usually  of  definite  number  and  united  into  a  compound 
pistil.  The  former  family  is  one  of  the  most  clearly 
defined  of  all  the  Angiosperms,  having  always  the  same 
number  of  parts  in  the  flower,  i.e.  four  sepals  and  petals, 
six  stamens,  and  two  carpels  (Fig.  51,  A,  B).  The 
poppies  are  more  variable  in  the  number  of  parts  in  the 
flower,  and  must  be  considered  as  a  more  generalized 
family  than  the  Cruciferse,  and  more  nearly  related  to 
the  Polycarpicse. 

The  sundews  and  pitcher-plants  (Fig.  58)  represent 
the  order  Cistiflorse,  and  are  distinguished  by  perfectly 
symmetrical  flowers,  but  are  of  most  interest  on  account 
of  their  extraordinarily  modified  leaves,  which  form  very 
efficient  insect-traps.  The  violets,  which  also  belong  to 
the  Cistiflorse,  are  characterized  by  their  showy,  strongly 
zygomorphic  flowers. 

Under  the  name  Eucyclse  have  been  included  a  large 
number  of  families  grouped  into  four  orders,  charac- 
terized by  usually  symmetrical  flowers  whose  parts  are 
in  fives.  Among  these  may  be  mentioned  the  vines 
(Vitacese),  maples  (Aceracese),  geraniums  (Gerania- 


210 


EVOLUTION   OF  PLANTS 


cese),  as  well  as  many  others  more  or   less   familiar. 
(Fig.  51,  C,  D.) 

The  order  Tricoccse,  of  somewhat  doubtful  affinity,  in- 
cludes the  single  family  Euphorbiacese  with  the  various 
species  of  Euphorbia  as  types.  A  few  are  cultivated, 
like  the  familiar  castor-bean  (Ricinus),  and  the  showy 


FIG.  51  (Higher  Choripetalre) .  —  A,  wall-flower  (Cheiranthus) :  the  parts 
of  the  flower  are  definite  in  number ;  B,  the  six  stamens,  the  two  outer 
ones  shorter  than  the  others,  and  the  pistil,  car,  made  up  of  two  cohe- 
rent carpels ;  C,  flower  of  Oxalis,  the  parts  perfectly  symmetrical,  and 
in  fives;  D,  the  ten  stamens,  an,  in  two  sets  of  five  each,  and  the  five 
carpels,  st ;  E,  flower  of  a  Spiraea,  one  side  removed  to  show  the  five 
free  carpels,  car,  and  the  numerous  stamens  inserted  upon  the  calyx 
margin;  F,  flower  of  the  common  pea  (Pisum),  showing  marked  zygo- 
morphy ;  G,  the  ten  stamens,  one  of  them  free,  and  the  single  carpel,  car, 
of  the  pea ;  H,  a  flower  of  Fuchsia,  with  "  inferior  "  ovary,  o,  and  showy 
colored  sepals. 

Crotons  and  Poinsettia  of  the  greenhouses.  The 
flowers  in  all  Euphorbiacese  are  inconspicuous,  but  it 
is  common  for  them  to  develop  showy  bracts  about  the 
clusters  of  flowers,  and  these  serve  the  same  purpose  as 
the  showy  petals  of  other  Choripetalse. 

The  most  specialized  as  well  as  the  most  numerous 


DICOTYLEDONS  211 

of  the  Choripetalse  are  the  Calycifloras,  so  called  from 
the  fact  that  the  sepals  are  united  into  a  tubular  or 
cup-shaped  calyx  upon  whose  margin  are  inserted  the 
petals  and  stamens.  Very  commonly  the  floral  axis  is 
prolonged  into  a  tube  which  may  be  completely  grown 
to  the  ovary  at  its  base,  so  that  the  ovary  becomes  "  in- 
ferior," as  we  have  seen  to  be  the  case  in  the  higher 
Monocotyledons.  Much  the  commonest  number  for 
the  sepals  and  petals  is  five,  although  some  families 
show  regularly  four,  e.g.  the  Onagraceae  (fuchsia, 
evening-primrose,  etc.),  and  occasionally  the  number 
is  indefinite  (Cactacese).  The  number  of  stamens  in 
the  Calyciflorse  is  occasionally  the  same  as  the  petals, 
but  usually  either  double  the  number,  or  still  more 
numerous. 

The  order  Rosiflorae,  which  is  subdivided  into  several 
families,  is  one  of  the  largest  and  most  familiar  groups 
of  the  Calyciflorse.  In  some  of  these,  e.g.  the  straw- 
berry, the  carpels  are  quite  separate,  while  in  others, 
e.g.  apple  and  pear,  they  are  more  or  less  completely 
united,  and  there  is  an  approach  to  an  inferior  ovary. 

The  myrtle  family  (Myrtacese)  is  mainly  tropical. 
The  petals  are  often  wanting,  but  the  numerous  sta- 
mens, which  are  white  or  red,  are  very  conspicuous  and 
serve  to  attract  insects  just  as  showy  petals  would  do. 
The  ovary  is  here  inferior,  and  the  tissues  of  the  calyx 
may  become  fleshy  and  edible  in  the  ripe  fruit,  as  in 
the  pomegranate  or  guava. 

The  Aralia  family  and  the  parsley  family  (Umbelli- 
ferse)  are  two  related  families  of  the  Calyciflorse,  which 
are  not,  however,  very  clearly  related  to  the  others. 
Of  the  former  the  common  ivy  (Hedera)  and  the  sev- 


212  EVOLUTION   OF  PLANTS 

eral  native  species  of  Aralia,  including  the  ginseng,  may 
be  mentioned.  The  Umbelliferse  are  mainly  inhabitants 
of  the  northern  hemisphere  and  are  all  closely  related. 
Both  of  these  families  are  distinguished  by  the  arrange- 
ment of  their  usually  inconspicuous  flowers  in  umbels 
—  hence  they  are  united  with  the  allied  family  Corna- 
QQdd  (dogwoods)  into  a  common  order  Umbelliflorse. 

Two  of  the  most  specialized  orders  are  the  Passi- 
florinse  (passion-flowers  and  their  allies)  and  the  Cac- 
taceee.  The  latter  is  a  very  peculiar  group  of  American 
desert  plants ;  the  former  are  also  largely  American, 
but  belong  principally  to  the  moist  tropical  regions. 

The  last  order  of  the  Calyciflorse  is  a  very  important 
one,  the  Leguminosse,  including  the  beans,  peas  (Fig. 
51,  F,  G),  and  other  leguminous  plants.  Of  the  three 
families,  two,  the  Mimosese,  of  which  various  species  of 
Acacia  and  Mimosa  are  cultivated,  and  the  Csesalpinese, 
of  which  the  honey-locust  (Gleditschia)  and  the""red-bud 
(Cercis)  may  be  mentioned  as  native,  are  mainly  tropi- 
cal, while  the  other  and  much  larger  family,  Papiliona- 
ceee,  includes  most  of  the  numerous  Leguminosse  of 
temperate  regions.  The  characteristic  butterfly-shaped 
flowers  of  these  plants,  and  their  pod-shaped  fruits,  are 
too  familiar  to  need  further  description. 

THE  SYMPETAL.E 

The  Dicotyledons  which  have  just  been  considered 
either  have  the  petals  entirely  separate  or  quite  absent. 
There  is  a  second  division,  including  the  most  specialized 
as  well  as  the  larger  number  of  the  Dicotyledons,  in 
which  with  very  few  exceptions  the  petals  are  more  or 


DICOTYLEDONS 


213 


IDC 


less  completely  united,  and  the  corolla  is  "  sympetalous," 
or  "gamopetalous."  The  greater  number  of  these,  in 
addition  to  their  being  more  highly  specialized,  indicate 
that  they  are,  as  a 
whole,  a  later  and 
more  differentiated 
group  than  the  Cho- 
ripetalse,  although  it 
must  be  remembered 
that  certain  families 
of  the  latter  are 
highly  specialized. 
The  highest  of  the 
Sympetalse,  how- 
ever, are  probably 
the  most  recent  and 
highly  developed  of 
all  plants. 

The  Sympetalae 
fall  readily  into  two 
main  divisions,  the 
Isocarpse,  which 
have  the  carpels 
equal  in  number  to 
the  petals,  and  the 

Anisocarpse,  in  which  they  are  fewer.  None  of  the  Sym- 
petalse  ever  have  the  carpels  separate,  but  they  are 
always  completely  united  into  a  compound  pistil.  The 
Isocarpae  are  supposed  to  be  the  more  primitive  of  the 
two  divisions,  and  a  few  of  them  have  the  petals  almost 
free  (Fig.  52,  C),  and  to  some  extent  connect  the  Cho- 
ripetalee  and  Sympetalse.  Of  these  isocarpous  forms  the 


FIG.  52  (Sympetalre).—  A,  flower  of  wild 
Azalea  (A.  viscosd) ,  one  of  the  isocarpous 
Sympetalae  ;  all  the  parts  of  the  flower  in 
fives ;  B,  a  section  of  the  ovary  of  Azalea, 
showing  the  five  divisions ;  C,  flower  of 
the  pine-sap  (Monotropa),  the  petals  and 
sepals  quite  separate ;  D,  flower  of  the 
sorrel-tree  (Oxydendrum),  the  petals  co- 
herent almost  to  the  tips ;  E,  flower  of 
shooting-star  (Dodecatheon) ,  one  of  the 
primrose  family;  F,  flower  of  Petunia, 
one  of  the  anisocarpous  Sympetalae; 
parts  of  the  flower  in  fives,  except  the 
two  carpels,  shown  in  the  cross-section 
of  the  ovary,  G. 


214  EVOLUTION   OF   PLANTS 

beautiful  Rhododendrons  and  Azaleas  (Fig.  52,  A)  are 
familiar,  as  well  as  the  various  species  of  cranberries, 
huckleberries,  wintergreen,  etc.  The  trailing  arbutus 
of  the  Atlantic  States,  and  the  manzanita  and  madrono 
(Arbutus)  of  the  Pacific  coast  are  also  characteristic 
types.  The  other  two  orders  of  the  Isocarpse  are  repre- 
sented by  the  primroses  (Primulinse)  and  the  persim- 
mons (Diosporinse). 

The  great  majority  of  the  Sympetalse  belong  to  the 
second  division,  Anisocarpse.  These  are  especially 
abundant  in  the  tropics,  where  they  form  the  predomi- 
nant constituents  of  the  vegetation.  The  less  specialized 
types  are  included  in  the  order  Tubiflorse,  with  regular 
tubular  or  funnel-shaped  flowers.  Here  belong  the  morn- 
ing-glories, the  phloxes,  and  nightshades,  all  of  them 
including  familiar  wild  or  garden  plants  (Fig.  52,  F). 

The  second  order  of  the  Anisocarpse,  the,  Labiati- 
florse,  as  the  name  indicates,  has  flowers  which  are  usu- 
ally strongly  bilabiate,  i.e.  are  markedly  zygomorphic. 
This,  together  with  a  reduction  in  the  number  of  sta- 
mens, indicates  a  more  specialized  type  than  the  Tubi- 
florse.  The  two  most  important  families  of  the  temper- 
ate regions  are  the  figworts  (Scrophulariacese)  and  mints 
(Labiatse),  both  of  which  include  numerous  familiar  wild 
and  cultivated  plants  (Fig.  53,  A-D).  In  both  of  them 
the  stamens  are  reduced  to  two  or  four,  and  they  often 
exhibit  very  perfect  adaptation  to  cross-fertilization. 
Allied  to  these,  and  represented  in  the  warmer  parts 
of  the  United  States  by  a  few  examples,  is  the  Bigno- 
nia  family,  much  more  abundant,  however,  in  tropical 
regions.  Catalpa  and  Tecoma  (the  trumpet-creeper) 
are  the  genera  occurring  within  our  limits. 


DICOTYLEDONS 


215 


The  third  order  of  the  Anisocarpse,  the  Contorts, 
includes  several  characteristic  families,*  among  them 
the  milkweeds  (Asclepiadacese)  and  dogbanes  (Apo- 
cynacese).  To  the  latter  belong  the  oleander  and 
periwinkle,  while  the  gentians,  and  the  olive  family 


05C 


FIG.  53  (Sympetalse,  Labiatiflorse,  Composite) .  —  A,  flower  of  dead-nettle 
(Lamium),  the  flower  strongly  zygomorphic ;  B,  stamens  and  pistil  of 
Lamium;  C,  flower  of  speedwell  (Veronica),  the  stamens  reduced  to 
two;  D,  flower  of  toad-flax  (Linaria),  the  flower  zygomorphic,  and 
the  base  of  the  corolla  prolonged  into  a  spur ;  E,  inflorescence  of  the 
Canada  thistle  (Cirsium),  the  small  flowers  aggregated  into  a  head 
which  looks  like  a  single  flower;  F,  an  individual  flower  of  E;  o,  the 
inferior  ovary;  p,  the  hairs  which  form  the  "pappus,"  or  calyx;  an, 
the  coherent  anthers;  G,  inflorescence  of  the  may-weed  (Maruta),  the 
outer  flowers  sterile  and  petal-like,  serving  merely  as  organs  for  attract- 
ing insects ;  H,  one  of  the  tubular  perfect  flowers  from  the  central  part, 
or  disk,  of  the  inflorescence  of  Maruta ;  I,  a  single  flower  from  the  inflo- 
rescence of  the  dandelion  :  all  the  flowers  are  alike  and  have  the  corolla 
split  open  and  strap-shaped  ;  p,  the  feathery  pappus ;  o,  the  ovary. 

(Oleacese),  with  the  lilac  and  ash  as  familiar  representa- 
tives, also  belong  to  the  Contortse. 

The  highest  of  all  the  Anisocarpse,  and  therefore  at 
the  head  of  the  whole  vegetable  kingdom,  are  the 
Aggregate,  including  several  families.  Of  the  lower 
families,  the  honeysuckles  (Caprifoliaceae)  and  the 
madder  family  (Rubiacese)  are  the  best  known,  while 


216  EVOLUTION   OF  PLANTS 

the  great  family  of  Composite  is  the  highest  of  all. 
The  latter  is  "the  largest  family  of  plants  and  shows 
extreme  specialization  of  the  floral  structures  in  the  ex- 
tensive cohesion  of  the  parts,  which  extends  to  the  sta- 
mens as  well  as  the  other  parts  of  the  flower.  The 
flowers,  as  is  well  known,  are  aggregated  in  dense  heads 
surrounded  by  bracts  which  give  the  whole  inflorescence 
the  appearance  of  a  single  flower  (Fig.  53,  E,  G).  This 
is  especially  so  in  such  forms  as  the  asters  and  daisies, 
where  the  outer  flowers  have  the  corolla  large  and  flat- 
tened, so  that  each  of  these  "  ray -florets  "  looks  like  a 
single  petal.  In  many  genera  these  outer  flowers  are 
destitute  of  stamens  and  sometimes  the  pistil  is  also 
abortive,  and  the  ray-florets  serve  simply  to  make  the 
inflorescence  conspicuous.  There  are  many  interesting 
transitions  between  the  lower  Composite,  where  all  the 
flowers  of  the  inflorescence  are  alike,  and  those  in  which 
the  ray-florets  are  entirely  sterile. 

The  type  of  inflorescence  developed  in  the  Composite 
seems  to  have  been  particularly  effective,  as  these  plants 
are  notoriously  prolific.  The  actual  number  of  seeds  is 
not  excessively  large  as  compared  with  many  other 
plants  ;  but  each  individual  flower  almost  always  suc- 
ceeds in  ripening  its  seed,  and  the  one-seeded  fruits  are 
usually  provided  with  most  efficient  means  of  transpor- 
tation. One  has  but  to  think  of  the  legions  of  common 
Composites,  —  daisies,  sunflowers,  thistles,  burdocks, 
dandelions,  and  many  others  of  our  commonest  and 
most  troublesome  weeds,  —  to  realize  how  well  fitted 
these  plants  are  to  hold  their  own  in  the  struggle  for 
existence. 


DICOTYLEDONS  217 

SUMMARY 

Owing  to  the  enormous  number  of  Dicotyledons  the 
task  of  arranging  them  systematically  is' a  formidable 
one,  and  it  is  unlikely  that  any  arrangement  yet  pro- 
posed can  be  considered  final.  Very  much  more  infor- 
mation is  needed  in  regard  to  the  development  of  both 
floral  and  vegetative  parts,  as  well  as  in  regard  to  the 
embryology,  especially  in  the  obscure  and  doubtful  types, 
before  we  shall  have  the  data  necessary  for  a  satisfactory 
classification.  Their  relation  to  the  Monocotyledons  is 
also  very  uncertain,  and  a  thorough  comparison  of  the 
lower  types  of  Dicotyledons  with  these  is  very  much 
needed. 

It  is  generally  admitted  that  the  apocarpous  Choripet- 
alse,  i.e.  those  with  entirely  distinct  carpels,  are  the  most 
primitive.  The  simpler  Ranunculaceae  offer  many  resem- 
blances to  the  apocarpous  Monocotyledons,  and  it  is  possi- 
ble that  here  we  have  a  point  of  contact  between  the  two 
groups.  It  is,  however,  not  at  all  impossible  that  the 
Dicotyledons  have  had  a  multiple  origin,  and  the  possi- 
bility of  the  derivation  of  the  Piperinese,  and  possibly  the 
lower  Centrospermae  from  monocotyledonous  types  like 
the  aroids,  is  not  improbable.  This  would  imply  that 
some,  at  least,  of  the  apetalous  Dicotyledons  are  prim- 
itive types,  and  not  reduced  from  petaloideous  forms. 
The  occurrence  of  numerous  apetalous  types  among  the 
oldest  fossil  remains  of  Dicotyledons  also  strongly  sug- 
gests their  primitive  character.  If  this  view  is  correct, 
it  is  quite  possible  that  the  Amentacese  and  some  other 
Apetalse  constitute  a  line  of  development  entirely  dis- 
tinct from  that  of  the  petaloideous  forms. 


218  EVOLUTION   OF   PLANTS 

In  passing  from  the  lower  to  the  higher  types  there 
is  a  reduction  in  the  number  of  parts,  accompanied  by 
their  cohesion.  The  carpels  are  first  affected,  and  then 
the  sepals,  and  finally  the  corolla.  Reduction  in  the 
number  of  stamens  is  common  in  certain  groups,  espe- 
cially the  Labiatiflorse,  and  the  cohesion  of  the  stamens 
among  themselves  occurs  regularly  in  the  Composite,  but 
is  much  less  perfect  than  that  of  the  other  floral  parts. 
Most  of  the  more  specialized  types,  both  of  Choripetalse 
and  Sympetalse,  have  inferior  ovaries. 

The  Sympetalse  are  unquestionably  the  highest  of  the 
Angiosperms.  Whether  the  group  is  a  homogeneous 
one,  or,  as  seems  more  likely,  sympetaly  has  originated 
more  than  once,  must  be  determined  by  further  re- 
searches. The  radially  symmetrical  (actinomorphic) 
Isocarpse  are  probably  nearer  the  Choripetalse,  as  shown 
by  the  occurrence  of  forms  like  Monotropa,  which  have 
sepals  and  petals  quite  distinct.  It  has  recently  been 
suggested  that  the  Isocarpse  have  perhaps  been  derived 
from  the  Centrospermse  among  Choripetalse  and  have 
given  rise  to  the  Tubiflorse  and  Labiatiflorse,  whose 
highest  members  are  the  mints  and  figworts.  Another 
line,  originating  from  the  Calyciflorse,  has  through  the 
Umbelliferse  developed  the  Rubiacese  (madder  family), 
and  through  these  the  Composite.  There  are  several 
less  important  lines  of  development  which  cannot  be 
taken  up  here,  and  it  must  be  remembered  that  the 
suggestions  given  here  as  to  the  origin  of  the  different 
groups  of  Dicotyledons  are  likely  to  be  essentially 
modified  when  we  are  in  possession  of  data  more  com- 
plete than  we  now  possess.  The  accompanying  diagram 
illustrates  graphically  the  arrangement  of  the  principal 
groups  of  Dicotyledons  adopted  here. 


DICOTYLEDONS 


219 


Compositse 


Labiatiflorae 


Sympetalse 
(Isocarpse) 


Sympetalse 
(Anisocarpse) 


Poplars,  willows,  etc. 
(Amentacese) 


Umbelliflorse 


Centrospermse  Calyciflorse 


Peppers 
(Piperinese) 


Polycarpicse 


Aroidese 


Apocarpse 


Monocotyledons 

Diagram  to  show    the   relationships   of   the  principal  groups  of 
Dicotyledons. 


CHAPTER   XII 

GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION 

THE  history  of  the  Plant  Kingdom  as  revealed  by  the 
geological  record  is  necessarily  very  fragmentary,  but 
nevertheless  the  study  of  fossil  plant  remains  has  yielded 
most  important  evidence  for  tracing  the  succession  of 
plant  forms.  The  record  is  most  unsatisfactory  with 
reference  to  the  lower  plants,  whose  delicate  tissues  are 
poorly  fitted  to  leave  recognizable  remains  in  the  rocks. 
Long  before  there  is  any  absolute  evidence  of  the  ex- 
istence of  plants,  it  must  be  assumed  that  these  lower 
plants  were  present  upon  the  earth,  but  naturally  their 
delicate  and  extremely  perishable  structures  have  left  no 
fossil  traces.  Indeed,  throughout  the  Thallophytes,  with 
few  exceptions,  the  fossil  remains  are  so  imperfect  that 
a  satisfactory  estimate  of  their  real  nature  is  often  quite 
impossible. 

The  ferns  and  their  allies  have  been  preserved  in  many 
cases  with  remarkable  perfection,  and  the  same  is  true 
of  many  flowering  plants,  especially  in  the  later  forma- 
tions, and  among  the  Algse  a  few  groups  possessing 
silicious  or  calcareous  cell-walls,  have  been  preserved 
in  a  recognizable  form,  but  these  nearly  all  belong  to 
the  later  formations  and  throw  no  light  upon  the  char- 
acter of  the  earliest  forms.  Among  the  vascular  plants, 
however,  the  tissues  are  sometimes  preserved  with  such 

220 


GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION     221 

remarkable  perfection,  that  thin  sections  of  them  may 
be  examined  with  the  microscope  and  reveal  the  mi- 
nutest details  of  their  cellular  structure.  A  careful 
study  of  such  remains  has  thrown  much  light  upon  the 
real  nature  and  affinities  of  many  fossil  types.  Very 
rarely,  it  is  true,  have  the  reproductive  parts,  so  essen- 
tial in  classification,  been  preserved ;  but  occasionally 
this  occurs,  and  a  study  of  these  has  been  of  the 
greatest  value  in  determining  the  relationship  of  these 
fossil  forms. 

Unfortunately,  too  much  of  the  work  upon  fossil 
plants  has  been  done  by  men  who  were  not  botanists 
and  who  were  not  sufficiently  acquainted  with  the  exist- 
ing plants  allied  to  the  fossil  ones.  Consequently  great 
confusion  has  arisen  in  the  attempts  to  name  and  classify 
these  fossils. 

In  general  it  may  be  said  that  the  geological  record 
bears  out  the  conclusions  reached  from  a  study  of  com- 
parative morphology,  although  as  regards  the  Thallo- 
phytes  the  record  is  too  imperfect  to  have  much  value. 

The  earliest  recognizable  plant  remains  occur  in  the 
lower  Silurian  rocks,  where  there  have  been  found  im- 
pressions which  have  been  referred  to  algse,  perhaps 
related  to  the  coarser  red  or  brown  forms  existing  at 
present,  but  not  readily  assignable  to  any  existing 
types,  so  that  the  real  nature  of  these  plant  remains,  if 
such  they  really  are,  is  exceedingly  doubtful. 

Of  the  existing  types  of  algoe,  a  number  are  known 
in  a  fossil  state,  but  seldom  from  the  earlier  rocks.  Of 
the  green  algse,  certain  Siphonere  occur  fossil  in  large 
numbers  from  the  Permian  rocks  upward.  These  plants, 
like  many  existing  ones,  were  heavily  encrusted  with 


222  EVOLUTION   OF   PLANTS 

lime  and  seem  to  have  played  an  important  r61e  in  rock- 
building.  That  anomalous  group,  the  Characese,  is  also 
represented  in  the  later  formations  by  a  considerable 
number  of  unmistakable  forms.  These  too,  owe  their 
preservation  to  the  calcareous  deposit  in  their  cell-walls. 
The  Characese  are  represented  not  only  by  fragments  of 
stems,  but  also  by  the  curious  spore-fruits,  which  are 
exactly  like  those  of  the  living  types.  The  earliest  of 
these  Characese  occur  in  the  Miocene  rocks.  Certain 
red  algse,  Corallinese,  are  abundant  in  the  Mesozoic 
rocks,  and  probably  occurred  in  the  later  Palseozoic  for- 
mations. Many  of  these  are  referable  to  existing  genera, 
and  closely  resemble  forms  which  are  still  living.1 

Among  the  Algse,  one  group,  the  Diatoms,  have  left 
very  abundant  remains,  but  as  yet  these  have  been  found 
only  in  the  more  recent  strata.  As  the  silicious  shells 
of  these  plants  are  very  permanent,  their  complete  absence 
from  Palseozoic  rocks  seems  to  indicate  that  the  group  is, 
comparatively  speaking,  a  recent  one.  The  deposits  of 
diatoms  are  extraordinarily  abundant  in  the  later  for- 
mations, the  first  ones  occurring  in  the  Mesozoic  rocks, 
where,  however,  they  are  much  less  abundant  than  in  the 
Tertiary  formations.  The  flinty  valves  or  shells  are 
perfectly  preserved,  and  make  their  identification  an 
easy  matter.  Many  of  the  genera  and  even  species  are 
identical  with  living  ones.  The  diatomaceous  deposits 
are  often  of  astonishing  thickness,  showing  that  these 
plants,  as  at  present,  occurred  in  enormous  masses 
together. 

The    fossil    fungi    are    too    few    and    imperfect    to 

1  The  most  recent  investigations  point  to  the  existence  of  Coralline 
algse  and  Siphonese  in  the  early  Silurian  deposits. 


GEOLOGICAL   AND    GEOGRAPHICAL   DISTRIBUTION     223 

throw  any  light  upon  the  origin  of  these  puzzling  organ- 
isms. 

While  it  is  reasonable  to  suppose  that  both  liver- 
worts and  mosses  existed  at  a  very  early  period,  their 
great  delicacy  has  prevented  their  preservation  as  fossils 
except  in  a  few  cases,  and  these  are  all  in  the  later  for- 
mations. No  certain  remains  of  Bryophytes  are  known 
from  the  Palaeozoic  rocks. 

With  the  Pteridophytes  the  case  is  very  different. 
From  the  Devonian,  and  possibly  still  lower,  their  re- 
mains occur  in  great  profusion,  especially  in  the  Car- 
boniferous rocks,  where  they  form  the  predominant  type 
of  vegetation,  and  their  remains  are  often  preserved  in  a 
most  perfect  manner,  even  the  inner  structure  often 
being  so  clear  as  to  make  a  comparison  with  the  tissues 
of  the  living  forms  an  easy  matter. 

The  earliest  remains  attributed  to  the  ferns  occur  in 
the  lower  Silurian  rocks,  where  a  fossil  named  Eopteris 
has  been  found.  It  is  doubtful,  however,  whether  this 
really  is  a  fern.  In  the  Devonian,  undoubted  ferns 
occur.  Some  of  these,  e.g.  Palseopteris,  are  admirably 
preserved  so  far  as  the  leaves  are  concerned,  and  some 
traces  of  sporangia  have  been  detected,  but  these  are  too 
imperfect  to  make  clear  the  affinity  of  the  plant  with 
modern  types. 

It  is  in  the  coal  measures  that  the  most  numerous 
remains  of  ferns  are  found,  and  many  of  these  are  in  a 
remarkably  perfect  state  of  preservation.  The  most 
recent  study  of  these  Carboniferous  ferns  shows  that  most 
of  them  are  eusporangiate,  and  evidently  related  to  the 
living  Marattiacese,  an  order  which  at  present  is  repre- 
sented by  a  small  number  of  tropical  species  which  are 


224  EVOLUTION  OF  PLANTS 

evidently  the  remnants  of  a  disappearing  type.  As  we 
have  endeavored  to  show  in  a  previous  chapter,  the 
primitive  nature  of  the  Marattiacese  is  also  shown  by 
the  structure  both  of  gametophyte  and  sporophyte.  The. 
Leptosporangiatse,  which  at  present  are  the  predomi- 
nant types  of  ferns,  are  absent  from  the  older  forma- 
tions, and  first  appear  with  certainty  in  the  early 
Mesozoic  rocks.  The  earliest  ones  belong  to  the  fami- 
lies which  are  nearest  the  Eusporangiatse,  while  the 
more  specialized  forms  appear  later. 

While  the  ferns  —  at  least  the  Leptosporangiates  — 
are  still  important  factors  in  the  present  vegetation  of 
the  earth,  the  other  two  orders  are  very  much  less 
prominent,  and  many  of  the  types  related  to  them  are 
now  quite  extinct.  Of  the  Equisetinese,  or  horsetails, 
only  the  genus  Equisetum  survives.  This  same  genus 
can  be  traced  back  to  the  Mesozoic,  and  possibly  even  to 
the  later  Palaeozoic  rocks,  where  it  is  associated  with 
many  peculiar  genera  which  disappear  completely  in  the 
later  formations.  Among  the  largest  and  best  known 
of  these  ancient  forms  are  the  species  of  Calamites, 
which  were  like  gigantic  horsetails,  and  whose  stems 
exhibit  a  secondary  thickening  of  the  vascular  bundles, 
SQ.  that  the  stem  continued  to  increase  in  size  until  the 
plant  assumed  tree-like  proportions.  Another  character- 
istic group  was  the  Annulariese,  a  peculiar  family  mainly 
restricted  to  the  Carboniferous  and  sometimes  associated 
with  Calamites.  In  the  few  cases  where  the  cones  of 
these  fossil  Equisetinese  have  been  preserved,  they  show 
an  arrangement  of  the  tissues  and  sporangia  much  like 
those  of  the  existing  species  of  Equisetum.  It  is  evi- 
dent that  some  of  these  ancient  Equisetinese  were  hete- 


GEOLOGICAL  AND   GEOGRAPHICAL  DISTRIBUTION    225 

rosporous,  but  the  difference  between  the  macrospores 
and  microspores  was  less  than  in  the  other  groups  of 
heterosporous  Pteridophytes. 

The  oldest  fossils  which  can  be  referred  to  the 
Equisetinese,  occur  in  the  Devonian  rocks.  They 
increase  in  numbers  in  the  overlying  formations,  reach- 
ing their  maximum  development  in  the  Carboniferous, 
after  which  they  rapidly  diminish  in  numbers,  until  the 
sole  survivors  of  this  once  important  group  are  reduced 
to  the  members  of  a  single  genus. 

A  very  characteristic  order  of  fossil  Pteridophytes 
is  the  Sphenophyllese,  sometimes  associated  with  the 
Calamiteee,  but  probably  better  separated  from  the  other 
Pteridophytes  as  a  special  class  now  totally  extinct. 
They  had  slender  stems  with  the  leaves  arranged  in 
whorls.  The  leaves  were  narrowly  spatulate,  with  more 
or  less  conspicuous  dichotomous  divisions  and  dichoto- 
mous  venation.  The  stem  was  traversed  by  a  single 
axial  vascular  bundle  not  unlike  that  of  Lycopodium. 
The  sporangial  spikes  have  been  preserved,  and  it  is 
evident  that  the  plants  were  sometimes  heterosporous. 
Their  exact  relation  to  the  other  Pteridophytes  is  still 
uncertain,  and  further  investigations  are  necessary  to 
determine  this. 

The  Lycopods  also  reached  their  greatest  develop- 
ment during  the  Carboniferous,  and  like  the  Equisetinese 
these  ancient  forms  far  surpassed,  both  in  size  and  com- 
plexity, their  modern  descendants,  none  of  which  are 
plants  of  large  size,  the  largest  being  slender  creeping 
or  half-climbing  forms,  reaching  occasionally  a  length 
of  four  to  five  metres.  The  living  genera,  Lycopodium 
and  Selaginella,  both  occur  fossil,  the  former  extending 


226  EVOLUTION  OF  PLANTS 

back  to  the  Devonian,  thus  being  one  of  the  oldest 
genera  known  in  the  whole  class.  During  the  Carbo- 
niferous there  appeared  numerous  arborescent  forms, 
the  principal  genera  being  Lepidodendron  and  Sigil- 
laria.  These  trees  exhibited  a  secondary  growth  of 
the  stem,  like  that  of  Conifers,  and  the  appearance 
of  these  was  probably  not  unlike  that  of  the  modern 
coniferous  trees,  suggesting  that  the  latter  may  be 
really  related  to  forms  like  Lepidodendron.  These  were 
heterosporous  like  Selaginella,  but  apparently  hetero- 
spory  was  not  so  pronounced. 

Both  of  the  lowest  existing  types  of  seed-bearing 
plants,  the  Cycads  and  Gingko,  are  found  fossil.  They 
probably  originated  in  the  later  Palaeozoic  formations, 
perhaps  in  the  later  Carboniferous.  After  the  Carbo- 
niferous they  increase  rapidly  in  numbers,  reaching 
their  maximum  in  the  Mesozoic,  after  which  they 
diminish  in  numbers,  giving  way  before  the  more 
specialized  Conifers  and  Angiosperms.  Many  of  the 
fossil  cycadean  remains  are  very  complete,  and  often 
are  obviously  much  like  the  existing  genera,  several  of 
which,  including  the  genus  Cycas,  undoubtedly  occur 
fossil. 

The  peculiar  genus  Gingko,  now  reduced  to  a  single 
species,  is  represented  by  numerous  fossil  species,  the 
oldest  dating  back  to  the  Permian. 

The  Cordaitese  comprise  a  group  of  entirely  extinct 
plants  which  have  been  considered  to  be,  in  many  ways, 
intermediate  between  the  Cycads  and  Conifers.  They 
are  most  abundant  in  the  coal  measures,  but  occur 
as  early  as  the  Devonian.  They  have  been  so  perfectly 
preserved,  in  some  instances,  that  the  structure  of  the 


GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION    227 

flowers  can  be  made  out,  even  to  the  inner  structure 
of  the  large  pollen-spores,  in  which  the  gametophytic 
structure  is  more  complicated  than  in  any  existing 
Gymnosperms.  The  pollen-spores  have  even  been 
detected  in  the  receptacle  above  the  opening  of  the 
ovule. 

It  is  doubtful  whether  any  true  Conifers  existed  prior 
to  the  Permian  formations,  where  forms  allied  to  living 
genera  occur,  but  no  existing  genera,  except  Gingko, 
which  probably  should  be  removed  from  the  Coniferse, 
occur  until  the  Mesozoic,  when  a  number  of  the  living 
types  are  encountered.  In  the  later  Mesozoic,  especially 
the  Cretaceous,  and  in  the  early  Tertiary  formations, 
they  become  abundant  and  characteristic  fossils,  some 
of  which  are  scarcely  distinguishable  from  living  forms. 
Most  of  the  existing  genera  are  represented  in  the 
Cretaceous  rocks,  and  in  some  cases  even  living  species 
can  be  recognized.  Thus  the  bald  cypress  of  the 
southern  United  States,  Taxodium  distichum,  is  repre- 
sented by  an  apparently  identical  form,  T.  disticJium 
miocenum,  which  is  a  common  and  widespread  fossil 
of  the  later  Miocene  and  early  Pliocene  rocks,  having 
been  evidently  far  more  widely  distributed  than  at 
present,  as  is  also  the  case  with  the  related  genera 
Glyptostrobus  and  Sequoia.  The  latter  genus  is  at 
present  reduced  to  two  species,  the  coast  redwood  and 
the  giant  tree,  confined  to  the  mountains  of  California, 
but  during  early  Tertiary  times  both  of  these,  as  well 
as  many  others,  were  common  trees  of  nearly  the  whole 
northern  hemisphere. 

The  pines  and  firs  appear  first  in  the  middle  of  the 
Mesozoic,  becoming  later  more  abundant,  and  holding 


228  EVOLUTION  OF   PLANTS 

their  own  in  modern  times  better  than  any  other  Coni- 
fers. At  present,  these  are  decidedly  the  prevailing 
types  of  coniferous  trees,  but  even  these,  in  most  re- 
gions, show  signs  of  yielding  to  the  encroachment  of 
the  angiospermous  trees. 

While  various  fossils  from  the  Carboniferous  and  early 
Mesozoic  formations  have  been  assigned  to  the  Angio- 
sperms,  it  is  not  until  the  later  Cretaceous  is  reached 
that  undoubted  remains  of  these  plants  occur.  Here 
are  found  unmistakable  traces  of  both  Monocotyledons 
and  Dicotyledons,  but  most  of  the  earliest  remains  are 
of  such  character  as  to  throw  little  light  upon  the 
relation  which  these  two  groups  bear  to  one  another. 
Among  the  earliest  forms  of  which  recognizable  re- 
mains are  found,  are  palms  and  screw-pines  among 
the  Monocotyledons,  and  birches,  willows,  and  poplars 
among  the  Dicotyledons.  It  need  not  necessarily  follow 
that  these  are  the  most  primitive  of  the  Angiosperms, 
although  they  are  doubtless  among  the  more  primitive 
types.  Their  preservation  is  simply  due  to  the  fact 
that  their  tissues  were  firm  and  resisted  decay  long 
enough  to  leave  clear  impressions.  Most  of  the  living 
Angiosperms  which  are  considered  as  probably  the  most 
primitive,  especially  among  the  Monocotyledons,  have 
extremely  delicate  and  perishable  tissues,  which,  as  in 
the  case  of  algse,  could  hardly  be  expected  to  leave 
recognizable  fossil  remains. 

In  the  later  Tertiary  rocks,  remains  of  Angiosperms 
became  very  abundant,  and  most  of  the  existing  groups, 
especially  of  Dicotyledons,  can  be  distinguished.  It  is 
evident  that  at  last  a  type  of  plants  has  been  evolved 
which  is  rapidly  crowding  out  the  less  perfect  types 


GEOLOGICAL   AND   GEOGRAPHICAL  DISTRIBUTION    229 

which  had  preceded  it.  While  the  comparative  scarcity 
of  Monocotyledons  has  been  explained  by  the  assump- 
tion that  they  are  a  later  development  than  the  Dicoty- 
ledons, a  more  plausible  explanation  is  probably  that  the 
small  number  of  types  in  which  the  tissues  were  firm 
enough  to  have  been  preserved,  accounts  for  their 
scarcity  in  a  fossil  state. 

The  geological  history  of  the  Dicotyledons  shows,  as 
might  have  been  expected,  that  the  earlier  types  are  all 
Choripetalse — largely  the  primitive  Amentaceoe,  willows, 
poplars,  etc.  These  may  have  been  preceded  by  herba- 
ceous forms  like  the  living  Piperineae,  but  of  these  no 
traces  have  been  found.  The  Sympetalse,  which  are 
the  most  specialized  and  numerous  group  at  present, 
do  not  appear  until  much  later,  and  the  fossil  record, 
so  far  as  it  goes,  is  quite  in  accord  with  the  conclu- 
sions based  upon  comparative  morphology. 

GEOGRAPHICAL  DISTRIBUTION 

In  considering  the  distribution  of  terrestrial  plants,  as 
they  at  present  exist,  many  factors  must  be  taken  into 
account.  First  of  all,  we  must  examine  the  original 
distribution  of  the  ancestors  of  the  existing  flora,  as 
revealed  to  us  by  the  study  of  fossil  forms.  There  are 
next  to  be  considered  the  factors  affecting  the  distri- 
bution of  these  forms  as  they  are  found  at  the  present 
time.  The  most  obvious  of  these  factors  are  climate, 
i.e.  temperature  and  rainfall ;  direction  of  mountains 
and  distribution  of  arid  districts;  currents  of  air  and 
water ;  animals,  including  man. 

The  distribution  of  plants  during  the  Tertiary  period, 


230  EVOLUTION   OF  PLANTS 

as  revealed  by  their  fossil  remains,  was  evidently  very 
different  from  that  of  the  present  time.  The  most  strik- 
ing point  about  these  Tertiary  fossils  is  the  wide  distri- 
bution of  many  types  now  extremely  limited  in  their 
range,  and  a  careful  study  of  the  question  leads  inevi- 
tably to  the  conclusion  that  at  this  period  in  the  earth's 
history  a  very  uniform  climate  must  have  prevailed 
over  a  large  part  of  the  northern  hemisphere,  and  cor- 
responding to  this  there  was  a  very  similar  flora  through- 
out its  whole  extent.  It  is  also  evident  that  a  very 
much  warmer  and  more  even  temperature  must  have 
prevailed  in  the  higher  latitudes  which  at  present  are 
incapable  of  supporting  any  but  the  hardiest  plants.  In 
early  Tertiary  times  palms,  sequoias,  magnolias,  sassa- 
fras, tulip-trees,  and  many  other  types,  now  quite  absent 
from  these  regions,  were  abundant  in  northern  Europe, 
and  even  in  Greenland  and  Siberia,  showing  conclusively 
that  at  that  time  a  very  much  milder  climate'must  have 
prevailed  there  than  at  present.  These  same  types  occur 
fossil  in  the  arid  western  United  States,  from  which  they 
have  completely  disappeared,  owing,  no  doubt  to  the  un- 
favorable conditions  now  existing. 

In  the  higher  latitudes  at  the  present  day,  there 
exists  a  zone  extending  round  the  earth,  where  the  cli- 
matic conditions  are  very  uniform,  and  where  the  plants 
are  very  similar  throughout,  much  as  was  the  case 
over  a  much  wider  zone  in  Tertiary  times ;  but  in- 
stead of  laurels  and  magnolias,  palms  and  sequoias, 
we  find  firs  and  willows,  birches  and  aspen-poplars. 
Many  northern  plants,  like  the  beautiful  little  Linnsea 
and  white  birch,  are  equally  at  home  in  Norway  and 
Canada,  and  the  reasons  are  the  same  which  governed 


GEOLOGICAL   AND   GEOGRAPHICAL   DISTRIBUTION    231 

the  distribution  of  the  Tertiary  flora  of  the  same  re- 
gions, i.e.  similar  climate  and  nearly  continuous  land 
communication. 

The  conditions  in  the  Antarctic  regions  are  very 
different  from  those  in  the  northern  hemisphere.  The 
southern  extensions  of  Africa  and  South  America  are 
widely  separated,  and  the  little  explored  land  area  sur- 
rounding the  pole  is  totally  shut  off  from  both  conti- 
nents, and  so  far  as  known  possesses  a  very  scanty 
flora,  both  on  account  of  its  isolation  and  the  excessive 
severity  of  the  climate. 

While,  as  we  have  seen,  the  flora  of  the  high  north- 
ern latitudes  is  very  similar  in  both  the  eastern  and 
western  hemispheres,  as  we  go  south,  more  and  more 
new  types  appear,  and  as  a  rule  these  are  quite  differ- 
ent in  the  Old  and  New  Worlds.  These  differences 
become  more  pronounced  as  the  tropics  are  approached, 
when  whole  orders  of  plants,  like  the  Cacti  and  Bro- 
meliaceye  of  the  New  World,  or  the  Proteacese  of  the 
Old  World  occur,  which  have  no  representatives  in  the 
other  hemisphere.  On  the  other  hand,  some  orders, 
like  the  Compositse  and  Leguminosse,  are  cosmopolitan, 
as  are  certain  genera,  but  very  few  species  are  thus 
widespread  except  as  they  may  have  been  distributed 
through  human  agencies,  so  that,  in  the  tropics  espe- 
cially, it  is  exceedingly  rare  to  find  identical  species  in 
the  Old  and  New  Worlds,  except  as  they  have  thus 
been  introduced. 

The  alpine  flora  of  high  mountains  usually  contains 
a  number  of  plants  often  identical  with,  or  closely  re- 
lated to,  species  growing  much  further  north,  but  not 
occurring  at  all  in  the  adjacent  lowlands.  This  is  es- 


232  EVOLUTION  OF  PLANTS 

pecially  noticeable  upon  lofty  mountains  in  the  tropics. 
Thus  in  Jamaica  the  writer  has  collected  upon  the 
highest  peaks  of  the  island  such  northern  plants  as 
strawberries  and  brambles,  buttercups  and  northern 
species  of  club-mosses,  none  of  which  occur  elsewhere 
in  the  island  nor  on  the  adjacent  mainland.  A  similar 
occurrence  of  northern  plants  upon  high  tropical  moun- 
tains has  been  repeatedly  observed.  The  presence  of 
these  northern  plants  on  the  summit  of  tropical  moun- 
tains has  been  explained  by  the  supposition  that  their 
ancestors  were  driven  south  by  the  advance  of  the  gla- 
cial ice-sheet,  and  with  the  retreat  of  the  latter,  and  the 
corresponding  increase  in  the  temperature  in  the  low- 
lands, they  retreated  up  to  the  cooler  regions  of  the 
mountain  summits,  not  being  able  to  live  in  the  hot 
lowlands. 

Where  an  extensive  chain  of  mountains  occurs,  run- 
ning north  and  south,  it  is  possible  to  see*  how  the 
northern  plants  follow  them,  ascending  higher  and 
higher  as  they  proceed  southward,  seeking  in  this  way 
the  same  climatic  conditions  they  have  left  behind  them. 
In  the  United  States  the  Appalachian  Mountains,  the 
Rockies,  and  the  ranges  of  the  Pacific  slope,  are  all 
beautiful  illustrations  of  this  method  of  distribution  of 
northern  plants. 

Comparing  the  north  temperate  regions  of  the  eastern 
and  western  hemispheres,  we  find  that  eastern  North 
America  much  more  nearly  resembles  eastern  Asia 
than  it  does  the  much  nearer  regions  of  western  Europe. 
The  latter  region  lies,  for  the  most  part,  much  further 
north  than  any  part  of  the  United  States,  and  being 
cut  off  from  the  south  by  high  mountains,  its  whole 


GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION    233 

flora  is  characteristically  northern.  The  regions  bor- 
dering on  the  Mediterranean,  of  course,  show  many 
forms  related  to  the  adjacent  regions  of  northern  Africa 
and  western  Asia. 

In  eastern  Asia  the  conditions  are  very  similar  to 
those  in  eastern  North  America.  In  both  regions  the 
main  trend  of  the  mountains  is  north  and  south,  so  that 
there  is  direct  communication  with  the  tropics,  and 
in  both  the  climatic  conditions  are  remarkably  similar, 
showing  great  extremes  of  heat  and  cold  in  the  northern 
portions,  the  characteristics  of  a  continental  climate. 
In  both  regions  a  very  large  area  lies  much  further 
south  than  Europe,  and  the  flora  is  much  richer, 
this  being  especially  noticeable  in  the  much  -larger 
number  of  forest  trees.  While  in  Europe  the  trees 
are  few  in  number,  probably  not  more  than  a  third  or 
fourth  as  many  as  in  the  United  States  or  eastern 
Asia,  in  the  two  latter  regions  there  is  a  remarkably 
large  number  of  types,  both  of  trees  and  herbaceous 
and  shrubby  plants,  which  are  absent  from  Europe, 
and  what  is  perhaps  most  unexpected,  absent  also  from 
the  Pacific  coast  of  North  America.  In  both  eastern 
Asia  and  Nm'th  America,  the  number  of  tropical  types 
is  very  much  larger  than  in  Europe,  where  very  few 
of  these  exist. 

The  interior  of  all  the  great  continents  except  Europe 
is  more  or  less  arid,  and  in  some  cases  extensive  deserts 
occur  with  a  very  peculiar  flora  adapted  to  the  desert  con- 
ditions. Similar  arid  conditions  prevail  in  the  warmer 
parts  of  western  Asia  and  America,  but  western  Europe, 
owing  to  the  invasion  of  the  land  by  branches  of  the 
sea,  and  the  influence  of  the  Gulf  Stream,  has  an  insular 


234  EVOLUTION   OF  PLANTS 

rather  than  a  continental  climate,  and  the  same  is  true 
of  the  northern  parts  of  the  Pacific  coast  of  North 
America. 

Within  the  tropics  there  are  few  genera  common  to 
the  Old  and  New  Worlds,  although  many  families,  e.g. 
palms,  orchids,  aroids,  and  others,  are  abundantly  repre- 
sented in  both  regions,  but  usually  by  distinct  genera. 
Where  a  genus  is  common  to  both  regions,  it  is  usually 
one  which  has  a  wide  range  through  the  north  tem- 
perate regions  as  well;  e.g.  the  orchidaceous  genus 
Cypripedium  and  many  genera  of  ferns,  e.g.  Polypodium, 
Adiantum. 

The  flora  of  isolated  regions,  seen  in  its  most  extreme 
form  in  such  oceanic  islands  as  the  Hawaiian  Islands 
and  St.  Helena,  is  always  exceedingly  peculiar,  owing  to 
the  long  intervals  at  which  new  forms  are  introduced  and 
the  modifications  which  most  of  these  subsequently  have 
undergone  on  account  of  their  changed  environment. 
Such  regions  always  contain  a  large  proportion  of  en- 
demic or  peculiar  species.  While  wide  expanses  of 
ocean  offer  the  most  effective  barriers  to  the  distri- 
bution of  most  plants,  high  mountains  and  deserts  also 
act  as  efficient  checks  to  the  migration  of*  plants,  and 
a  very  different  flora  may  exist  upon  opposite  slopes 
of  high  mountain  ranges  separated  by  only  a  few  miles 
of  actual  distance.  A  marked  instance  of  this  is  seen 
in  the  character  of  the  plants  upon  the  eastern  and 
western  slopes  of  the  Andes.  In  the  United  States 
the  almost  totally  different  character  of  the  plants 
of  the  Atlantic  and  Pacific  coasts,  except  in  the  north- 
ern regions  where  occur  a  number  of  the  sub-polar 
types  common  to  the  whole  northern  zone,  illustrates 


very  graphically  the  effect  of  the  great  central  arid 
region  and  lofty  mountains  in  preventing  the  migration 
of  plants  from  one  side  of  the  continent  to  the  other. 

As  we  have  already  intimated,  it  is  evident  from  the 
geological  record  that  in  Tertiary  times  the  northern 
regions  enjoyed  a  much  milder  climate  than  at  present, 
this  being  shown  by  the  character  of  the  fossil  remains  of 
both  animals  and  plants.  Many  of  the  common  Tertiary 
types  of  plants  are  now  represented  by  a  small  number 
of  their  descendants  restricted  to  a  much  smaller  range, 
like  the  species  of  Torreya  and  Sequoia.  In  Europe 
we  find  these  forms  associated  with  many  others,  like 
the  magnolias,  tulip-trees,  hickories,  and  many  more, 
still  existing  in  eastern  Asia  and  America,  but  else- 
where extinct.  In  short,  the  Tertiary  flora  of  the  sub- 
polar zone  was  made  up  mainly  of  types  still  existing 
in  regions  much  further  south.  The  modern  descend- 
ants of  these  Tertiary  plants  have  many  of  them  per- 
sisted unchanged  in  some  regions,  but  have  been  quite 
crowded  out  or  very  much  modified  in  others.  The 
retreat  of  these  plants  from  their  northern  habitat  was 
mainly  due,  no  doubt,  to  great  climatic  changes,  and 
the  principal  of  these  was  the  severe  glaciation  to  which 
the  whole  northern  part  of  the  globe  was  subjected  in 
post-tertiary  times. 

As  the  ice-sheet  advanced  southward,  the  plants  were 
driven  before  it,  and  many  forms  were  doubtless  com- 
pletely destroyed.  The  fate  of  these  progenitors  of  the 
existing  flora  of  the  northern  hemisphere  was  very  dif- 
ferent in  different  parts  of  the  earth.  In  America  and 
eastern  Asia  the  trend  of  the  main  mountain  ranges  is 
north  and  south,  and  offered  no  barrier  to  the  south- 


236  EVOLUTION   OF   PLANTS 

ward  retreat  of  vegetation  before  the  advancing  ice- 
sheet,  and  as  the  latter  retired  again  the  plants  were 
enabled  to  return  northward. 

In  Europe,  owing  to  the  position  of  the  great  moun- 
tain chains,  as  well  as  its  higher  latitude,  the  whole 
region  north  of  the  Alps  was  subjected  to  the  action  of 
the  glaciers,  and  the  southward  retreat  of  the  plants  being 
cut  off,  very  many  forms  perished,  while  the  same  plants 
have  survived  in  the  more  favored  regions  of  Asia  and 
America,  in  both  of  which  a  far  larger  number  of  sur- 
vivors of  the  primordial  Tertiary  flora  occur  than  in 
Europe.  The  occurrence  of  nearly  related  isolated  types 
in  widely  separated  regions  can  almost  always  be  ex- 
plained as  a  survival  from  once  widely  distributed  an- 
cestors. In  the  case  of  herbaceous  plants,  such  as 
Podophyllum,  Stylophorum,  and  other  peculiar  types 
common  to  eastern  Asia  and  Atlantic  North  America, 
we  can  only  reason  from  analogy,  but  in  the  case  of  many 
woody  plants,  especially  trees,  e.g.  the  tulip-tree  (Lirio- 
dendron),  Torreya,  etc.,  this  is  abundantly  proved  by  the 
fossil  remains. 

Perhaps  the  most  striking  instance  known  of  close 
correspondence  in  the  flora  of  widely  separated  regions, 
is  the  one  already  spoken  of,  i.e.  the  great  number  of 
identical  or  closely  related  plants  found  in  the  temper- 
ate regions  of  Pacific  Asia,  northern  China,  Mantchuria, 
and  Japan  —  and  Atlantic  North  America.  Much  of  our 
knowledge  of  these  extraordinary  similarities  we  owe  to 
the  labors  of  Asa  Gray. 

The  writer  recalls  vividly  the  strangely  familiar  aspect 
of  the  vegetation  of  Japan,  especially  in  the  northern 
part,  where  nearly  all  of  the  more  noticeable  plants 


GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION     237 

are  either  identical  with  or  closely  related  to  eastern 
American  species.  Such  characteristic  plants  as  the 
fox-grape,  poison  ivy,  and  other  sumachs,  bitter-sweet 
(Celastrus),  the  sensitive  fern  (Onocleasehsibilis*),  elms, 
maples,  beeches,  oaks,  and  magnolias,  very  close  to  their 
American  relatives,  as  well  as  others  familiar  to  the 
botanist,  were  the  predominant  features  of  the  vegeta- 
tion. Were  these  forms  also  common  to  Pacific  North 
America  and  continuous  across  the  continent,  there 
would  be  nothing  remarkable  in  their  occurrence  in 
Japan ;  but  most  of  them  are  entirely  absent  from  our 
Pacific  coast,  and  from  all  the  intermediate  country. 

The  list  of  forms  common  to  the  Mantchurian-Japan- 
ese  region  and  Atlantic  North  America  is  very  large, 
and  at  first  seems  impossible  to  explain ;  but  when  we 
consider  them,  as  they  doubtless  are,  remnants  of  a  once 
continuous  northern  flora,  which  have  survived  in  these 
two  widely  separated  areas  owing  to  very  similar  cli- 
matic conditions,  the  wonder  ceases. 

The  southern  United  States  illustrate  very  clearly 
the  very  different  character  of  plants  in  the  same  lati- 
tude, and  over  a  continuous  area,  due  to  different  condi- 
tions of  topography  and  rainfall.  The  southwestern 
United  States  —  southern  California  and  Arizona — show 
genuine  desert  conditions  with  an  extremely  character- 
istic flora,  of  which  cacti,  agaves,  yuccas,  sage-brush,  etc., 
are  the  conspicuous  features.  This  flora  is  closely  re- 
lated to  that  of  Mexico,  and  to  some  extent  to  that  of 
Pacific  South  America.  As  we  pass  eastward,  the  lofty 
ridge  of  the  Rocky  Mountains  forms  an  effective  barrier 
against  the  passage  of  some  forms,  and  the  heavier  rain- 
fall on  the  eastern  slopes  of  the  mountains,  increasing 


238  EVOLUTION  OF   PLANTS 

as  the  Gulf  of  Mexico  is  approached,  is  unfavorable  for 
the  growth  of  the  strictly  desert  plants.  With  the 
increase  in  the  rainfall,  the  desert  gives  place  to  a  prairie 
formation,  with  coarse  grasses  and  many  showy  flowers, 
phloxes,  evening  primroses,  and  gay  Composite.  Still 
further  east  the  amount  of  moisture  is  sufficient  for  the 
growth  of  a  few  low-spreading  trees  like  the  "  mesquit " 
of  the  Texan  plains,  and  in  eastern  Texas  open  forests 
of  pines  appear,  which  presently  give  way  to  the  dense 
forests  and  swamps  of  the  Gulf  region  of  Louisiana. 
Here  the  conditions  are  almost  tropical.  An  extremely 
heavy  rainfall  and  high  temperature  combine  to  produce 
a  rank  vegetation,  forming  deep  swamps  and  jungle-like 
forests.  Very  few  of  the  plants  occurring  in  these  wet 
forests  and  swamps  are  at  all  related  to  the  desert  plants 
of  the  same  latitude,  but  have  their  nearest  allies  among 
the  plants  of  the  Atlantic  side  of  the  continent.  The 
eastern  forests,  unlike  those  of  the  Pacific  slope,  contain 
few  Conifers,  but  there  is  a  remarkable  variety  of 
angiospermous  trees,  most  of  which  are  deciduous. 
Hickories,  gums,  magnolias,  tulip-trees,  elms,  beeches, 
and  many  other  trees,  quite  absent  from  the  Pacific 
coast,  are  important  constituents  of  this  magnificent 
forest  flora,  while  the  herbaceous  plants  associated  with 
them  are  quite  as  diverse  in  character.  Among  the 
plants  of  the  Gulf  region  are  a  number  of  genuine  tropi- 
cal types,  like  the  "  Spanish  moss,"  the  most  northerly 
representative  of  the  pineapple  family,  and  the  palmet- 
toes,  the  most  northerly  palms  of  the  United  States. 

California,  owing  to  its  peculiar  topography,  has  the 
most  varied  flora  of  any  region  within  the  United  States, 
and  is  extremely  interesting  in  respect  to  the  origin  of 


GEOLOGICAL  AND   GEOGRAPHICAL   DISTRIBUTION    239 

its  flora.  The  longitudinal  ranges  of  mountains  travers- 
ing the  state  form  part  of  a  continuous  series  of  lofty 
mountains  extending  from  Alaska  to  Mexico,  and  serve 
as  a  highway  for  the  migration  of  many  northern  types, 
which  are  thus  enabled  to  extend  their  range  far  beyond 
their  usual  southern  limit.  Thus  in  the  mountains  of 
northern  California  are  found  northern  genera  like 
Linncea,  and  violets  and  trilliums  like  those  of  the  north- 
eastern states.  With  these  occur  also  some  northern 
Asiatic  genera,  like  Fritillaria,  which  are  entirely  absent 
from  eastern  North  America.  In  the  northern  part  of 
the  state,  and  in  Oregon  and  Washington,  many  of  the 
plants  common  to  the  sub-polar  zone  occur  in  the  val- 
leys, but  further  south  they  ascend  the  mountains  and 
finally  disappear  entirely. 

The  most  remarkable  feature  of  the  Calif ornian  flora 
is  the  extraordinary  number  and  immense  size  of  the 
coniferous  trees.  Evidently  the  conditions  upon  the 
mountain  slopes  of  the  Pacific  coast  are  especially 
favorable  for  the  growth  of  these  ancient  trees.  A  very 
even  temperature,  with  heavy  precipitation,  has  appar- 
ently been  the  reason  for  the  survival  of  the  last  of  the 
great  Sequoias,  as  well  as  other  Conifers  not  found 
elsewhere. 

Southward,  the  rainfall  in  California  diminishes  very 
rapidly,  and  the  conditions  are  unsuited  to  plants  re- 
quiring much  moisture.  Consequently  we  find  the 
northern  plants  giving  way  to  immigrants  from  the  dry 
regions  south,  i.e.  from  Mexico  and  South  America.  To 
the  European  or  eastern  American,  the  aspect  of  the 
great  valleys  of  central  and  southern  California  is  very 
unfamiliar,  and  entirely  different  from  that  of  the  moun- 


240  EVOLUTION   OF  PLANTS 

tainous  districts  of  the  north.  Instead  of  dense  forests, 
with  an  undergrowth  of  dogwoods,  rhododendrons, 
brambles,  roses,  and  with  the  ground  carpeted,  with 
mosses  and  ferns,  we  find  the  floor  of  the  valleys  and 
the  rolling  foot-hills  covered  with  annual  grasses,  with 
which,  in  the  spring,  are  mingled  numberless  showy 
flowers,  unfamiliar  to  the  eastern  botanist  except  in 
gardens.  Fiery  Eschscholtzias,  blue  Nemophilas,  pink 
and  yellow  Mariposa  lilies,  and  numberless  other 
flowers,  make  masses  of  brilliant  color  of  unrivalled 
beauty.  Here  and  there  are  scattered  spreading  ever- 
green oaks,  and  on  the  hillsides  are  thickets  of  low- 
growing  shrubs,  "  chaparral,"  made  up  of  Manzanita, 
Ceanothus,  and  other  western  types,  while  the  streams 
are  bordered  with  beautiful  madronos  (Arbutus),  bay- 
trees,  and  big-leaved  maples,  as  well  as  the  more  fa- 
miliar alders  and  cottonwoods.  The  central  part  of 
the  state  is  the  meeting-ground  for  the  two  diverse 
floras,  the  northern  types  often  following  the  canons 
down  to  the  valleys,  where  they  mingle  with  the  south- 
ern flora. 

While  the  natural  conditions  of  topography  and 
climate  have,  of  course,  been  the  most  potent  factors 
in  the  present  distribution  of  plants,  animals  have  also 
played  an  important  part,  and  especially  man.  The 
advent  of  man  into  many  regions  has  quite  transformed 
them,  so  far  as  the  flora  is  concerned.  In  the  tropics 
many  of  the  most  characteristic  plants,  such  as  the 
banana,  breadfruit,  cocoa-palm,  and  mango,  as  well  as 
many  weeds,  like  the  sensitive  plant,  have  become  nat- 
uralized everywhere.  So  in  temperate  regions  many  in- 
troduced weeds  have  taken  possession  of  the  soil  to  the 


GEOLOGICAL   AND   GEOGRAPHICAL  DISTRIBUTION    241 

complete  exclusion  of  the  native  plants.  Originally  the 
whole  of  Atlantic  North  America  was  an  unbroken  for- 
est, with  an  undergrowth  of  delicate  shade-loving  plants. 
With  the  clearing  away  of  the  primeval  forest  these 
plants  quickly  perished,  and  a  host  of  foreign  weeds, 
grasses,  thistles,  dandelions,  docks,  plantains,  rushed 
in  to  occupy  the  waste  room.  As  civilization  pushed 
westward,  the  hordes  of  these  European  immigrants 
were  met  by  the  prairie  plants,  which  were  able  to  cope 
with  them  successfully,  so  that  now  the  farmer  has  to 
contend  with  two  sets  of  enemies,  the  European  weeds 
on  the  one  hand,  and  the  prairie  weeds,  rag-weed,  bur- 
marigold,  Rudbeckia,  sunflowers,  etc.,  on  the  other, 
These  weeds  are  transported  with  grain  in  railway 
cars,  or  cling  to  the  coats  of  animals  or  the  clothes  of 
human  beings,  and  in  these  days  of  rapid  transit,  plants* 
have  also  taken  advantage  of  the  improved  means  of 
travel. 

In  most  parts  of  California,  owing  to  the  long  dry 
season,  most  of  the  weeds  from  northern  Europe  do  not 
thrive,  and  instead  we  find  weeds  whose  home  is  upon 
the  shores  of  the  Mediterranean.  Probably  introduced 
by  the  original  Spanish  settlers,  wild  oats,  alfilaria, 
bur-clover,  and  other  south  European  plants  have  es- 
tablished themselves  in  the  sunny  valleys  of  California, 
where  they  grow  side  by  side  with  the  poppies  and 
lupines,  which,  however,  are  quite  able  to  hold  their 
own. 


CHAPTER   XIII 

ANIMALS  AND   PLANTS 

ANIMALS,  being  incapable  of  manufacturing  organic 
food  themselves,  are  necessarily  dependent,  directly  or 
indirectly,  upon  plants  for  their  supply  of  food ;  but, 
on  the  other  hand,  many  plants  depend,  more  or  less 
completely,  upon  animals  for  their  existence.  While 
these  are  usually  flowering  plants,  still  among  the  lower 
forms  of  plant  life  many  instances  might  be  cited, 
•especially  among  the  parasitic  fungi,  like  the  insect- 
fungi,  and  some  water-moulds.  The  same  may  be  said 
of  many  of  the  pathogenic  bacteria,  or  disease  germs. 

Occasionally  insects  appear  to  be  useful  to  certain 
fungi  by  scattering  their  spores.  Such  fungi  offer 
certain  means  of  attracting  insects,  either  in  the  form 
of  a  honey-like  secretion,  or  their  odor.  Thus  the 
evil  odor  given  off  by  some  f ungi,  especially  the  Phal- 
loidere,  attracts  carrion-loving  insects,  which  carry  away 
with  them  the  spores  which  are  imbedded  in  a  slimy 
substance  exuded  by  the  fungus. 

It  is  among  the  seed-plants,  however,  that  we  meet 
with  the  most  obvious  adaptations  connected  with  animal 
organisms.  The  development  of  edible  seeds  and  fruits 
in  so  many  plants  is,  in  most  cases,  directly  referable  to 
such  adaptation.  Where  the  seeds  themselves  are  edible, 
of  course  a  large  proportion  are  destroyed  by  the  ani- 

242 


ANIMALS   AND    PLANTS 


243 


mals  which  devour  them,  but  a  certain  number  of  the 

seeds  carried  away  are  not  eaten,  and  these  are  thus 

distributed  more  widely  than  would  be  the  case  were 

they  to  fall  to  the  ground  directly.     Where  the  seed 

itself  is  not  the  edible  part  of  the  fruit,  but  is  enclosed 

in  an  edible  pulp,  there  is  no  question  that  we  have 

to    do    with   a   case    of   special 

adaptation.     In  such  cases,  e.g. 

most  of  the  ordinary  cultivated 

fruits,  the  fleshy  edible  portion 

is  eaten  and  the  seeds  rejected. 

Or,  if  the  fruits  are  small,  the 

whole    fruit   is    eaten    and   the 

seeds    pass    uninjured    through 

the  body  of  the  animal.     Birds 

are  especially  important  agents 

in  the  distribution  of  seeds,  on 

account  of   the   long   distances 

over  which  they  travel. 

Another  method  of  distribu- 
tion of  seeds  and  fruits  through 


FIG.  54.  —  A,  spikelet  of  a  grass 
(Hordeum  murinuni),  the 
long  awn  furnished  with  re- 
curved barbs ;  B,  part  of  the 
awn  enlarged  to  show  the 


-large 

the  agency  of  animals  is  seen  in     barbs;  c,  fruit  of  bur-clover 

,,         ?        ,  ,,  ,,       (Medicago) ;    D,   four  spiny 

the   development  ot   organs   01     fruits    of    the   common 

i  -,  .-I  £      hound 's-tongue      (Cynoglos- 

attachment,  such  as  the  awns  of     sum). 
grasses,  the  hooks  and  barbs  de- 
veloped by  the  fruits  of  many  Composite,  Borragina- 
cea3,  etc.  (Fig.  54).     The  pedestrian  who  returns  from  a 
ramble  through  the  fields,  covered  with  a  varied  assort- 
ment of  "  burs,"  is  but  acting   as    Nature's   unwilling 
agent  in  the  distribution  of  her  plant  children.     Those 
plants  which  we  call  weeds  —  burdocks,  beggar's-ticks, 
hound's-tongue,  bur-clover  —  owe  their  success  in  the 


244  EVOLUTION   OF   PLANTS 

struggle  for  existence  largely  to  the  perfect  provision 
for  the  distribution  of  their  seeds  which  they  have  de- 
veloped. Every  animal  which  brushes  against  one  of 
these  plants  covered  with  its  ripe  fruits,  carries  away  its 
quota  of  seeds,  to  dislodge  them  far  away  from  the  place 
where  they  grew.  In  this  way  many  plants  have  been 
carried  from  their  European  home  to  all  quarters  of  the 
globe,  and  where  the  conditions  are  favorable,  have 
quickly  taken  possession  of  the  new  territory. 

The  extraordinary  variety  shown  by  the  flowers  of 
Angiosperms  is  intimately  associated  with  the  question 
of  cross-fertilization  through  the  agency  of  animals, 
mostly  insects ;  and  the  extraordinary  development  of 
certain  groups  of  insects  is  the  result  of  a  reciprocal 
adaptation. 

There  is  little  doubt  that  the  first  flowers  were  very 
simple,  probably  not  unlike  those  of  certain  low  types 
still  existing,  and  consisted  of  a  single  carpe"!  or  sta- 
men, or  perhaps  a  group  of  sporophylls,  without  any 
trace  of  the  showy  corolla  found  in  the  higher  forms. 
The  simple  flowers  of  the  aroids,  pond- weeds,  peppers, 
willows,  and  others  (Figs.  43,  45,  49),  are  examples  of 
such  flowers,  and  whether  this  simplicity  is  primitive  or 
secondary,  some  such  forms  must  have  been  the  starting- 
point  from  which  proceeded  the  development  of  the 
specialized  flowers  of  the  higher  groups  of  Angiosperms. 
Such  simple  flowers  are  usually  quite  dependent  upon 
chance  for  the  transfer  of  the  pollen-spores  to  the  stigma, 
and  with  the  exception  of  a  few  aquatic  forms,  the  agency 
by  which  this  is  effected  is  the  wind ;  hence  these  flow- 
ers are  called  "  anemophilous,"  or  wind-fertilized.  These 
anemophilous  flowers  are  always  inconspicuous  and  odor- 


ANIMALS   AND   PLANTS  245 

less.  In  such  plants  a  large  amount  of  pollen  is  neces- 
sary in  order  that  fertilization  may  be  insured,  as  a  very 
large  part  of  it  fails  to  reach  the  carpels.  An  extreme 
case  of  this  is  seen  in  the  pines  and  firs,  where  the 
amount  of  pollen  is  enormously  in  excess  of  what  is 
actually  needed  for  fertilization. 

The  development  of  contrivances  by  which  the  trans- 
fer of  pollen  to  the  pistil  is  facilitated,  results  in  an 
obvious  saving  of  pollen,  and  is  in  itself  an  advantage  ; 
but  experiment  has  demonstrated  that  cross-fertilization, 
i.e.  pollination  of  one  flower  by  pollen  from  another 
one,  is  generally  of  advantage  to  the  plant,  as  seed  so 
produced  is  usually  more  vigorous  than  when  the 
ovule  is  fertilized  by  pollen  developed  in  the  same 
flower. 

The  simpler  flowers  have  no  enveloping  leaves,  and 
the  first  step  toward  the  development  of  a  floral  en- 
velope or  perianth  was  probably  the  production  of 
small  scale-like  leaves,  either  green  or  membranaceous 
in  texture.  The  change  from  these  inconspicuous, 
purely  protective  floral  leaves,  to  those  which  are  more 
or  less  conspicuously  colored,  marks  the  next  advance 
in  the  evolution  of  the  flower.  This  bright-colored 
corolla  would  no  doubt  make  the  flower  more  conspic- 
uous, and  attract  insects  in  search  of  pollen.  Such  an 
insect  visiting  the  flower  would  be  pretty  sure  to  carry 
some  of  the  pollen  to  other  flowers  of  the  same  kind, 
insuring  cross-fertilization.  As  a  result  of  natural  se- 
lection, it  is  easy  to  conceive  how  flowers  having  the 
showiest  corollas  would  stand  a  better  chance  of  at- 
tracting insects  and  thus  being  cross-fertilized.  These 
plants  would  produce  a  greater  amount  of  seed,  and  in 


246 


EVOLUTION   OF   PLANTS 


time,   by   further   modifications    of   their   descendants, 
other  adaptations  for  cross-pollination  would  arise. 

There  are  many  genera,  especially  among  the  lower 
Dicotyledons,  which  exhibit  in  a  most  interesting  way 
all  gradations  between  inconspicuous  self-pollinated 
flowers,  and  showy  ones  dependent  upon  insects.  This 

is  shown,  for  example, 

A  „   /-,$7\      .       in  the  genus  Ranun- 

culus, which  includes 
the  various  species  of 
buttercup.  The  in- 
conspicuous R.  abor- 
tivus  is  entirely  in- 
dependent of  insect 
aid,  while  such  showy 
species  as  R.  acris  or 
R.  Californicus  are 
visited  freely  by  in- 
sects, although  they 
are  probably  not  en- 
tirely dependent  upon 
them  to  insure  fertili- 
zation. Similar  vari- 
ation is  found  in  the 
genus  Geranium. 

In  the  simplest  of  these  "  entomophilous  "  or  insect- 
fertilized  flowers,  such  as  the  buttercup  or  anemone 
(Fig.  55,  A),  the  flower  is  wide  open,  with  the  entirely 
separate  parts  arranged  radially,  and  often  indefinite 
in  number.  We  find  in  such  generalized  flowers  that 
the  variety  of  insects  visiting  them  is  large,  and  they 
are  seldom  incapable  of  self-pollination  in  case  insect 


FIG.  55. — A,  flower  of  Anemone  coronuria, 
the  petals  absent,  but  replaced  by  the 
showy  sepals,  s ;  B,  inflorescence  of  the 
dogwood  (Cornus  florida),  the  incon- 
spicuous flowers,  .#,  surrounded  by  four 
showy  bracts,  &;  C,  the  "calla-lily" 
(Richardia),  with  the  central  spike  of 
small  flowers  enclosed  by  the  large 
white  "spathe,"  sp. 


ANIMALS   AND   PLANTS  247 

visits  are  prevented.  In  more  highly  specialized  flow- 
ers the  parts  are  usually  so  modified  as  to  restrict  the 
insect  visitors  to  a  smaller  number;  in  extreme  cases 
often  a  single  species,  or  a  few  species  belonging  to  a 
single  genus.  Thus  in  the  buttercup  family  we  find  not 
only  the  generalized  type  of  flower  of  Ranunculus,  or 
Anemone,  but  the  highly  specialized  ones  of  the  colum- 
bine (Aquilegia)  (Fig.  50),  larkspur  (Delphinium),  and 
monk's-hood  (Aconitum).  In  these  the  parts  of  the 
flower  are  very  much  changed,  and  in  the  columbine 
and  larkspur  deep  nectaries  are  developed  which  are 
accessible  only  to  insects  with  long  tongues,  like 
bumblebees  or  butterflies;  or  in  the  case  of  the  scar- 
let-flowered columbines,  they  are  visited  by  humming- 
birds. It  is  interesting  to  note  that  in  these  extremely 
specialized  Ranunculacese  there  has  been  little  depart- 
ure in  the  number  of  parts  from  the  primitive  buttercup, 
and  all  the  parts  remain  quite  separate. 

It  sometimes  happens  that  the  flowers  themselves 
remain  inconspicuous,  but  are  grouped  together  with 
showy  colored  bracts  about  the  inflorescence,  and  these 
showy  leaves  serve  to  attract  insects  just  as  the  petals  of 
other  flowers  do.  Familiar  examples  of  this  are  seen  in 
the  common  "  calla-lily,"  (Fig.  55),  where  the  large  white 
spathe  surrounding  the  small  flowers  is  very  conspicu- 
ous, and  many  other  aroids,  such  as  species  of  Anthu- 
rium,  possess  these  showy  spathes.  Another  similar 
case  is  that  of  certain  species  of  Cornus,  like  the  beau- 
tiful dogwood  (<7.  florida,  Fig.  55,  B),  where  the  group 
of  small  flowers  is  surrounded  by  four  large  white 
bracts,  the  whole  looking  like  a  large  four-parted  flower, 
and  the  tree  when  in  bloom  is  exceedingly  showy, 


248  EVOLUTION  OF  PLANTS 

although  the  flowers  themselves  are  inconspicuous. 
Many  such  cases  occur  in  the  Euphorbia  family,  one 
of  the  most  familiar  being  that  of  Poinsettia,  a  common 
greenhouse  shrub,  having  the  flowers  surrounded  by 
numerous  large,  brilliant  red  bracts. 

We  have  already  seen  that  in  the  more  specialized 
types  of  flowers'  there  is  usually  a  reduction  in  the 
number  of  parts,  accompanied  by  a  tendency  to  a  coa- 
lescence of  the  members  of  each  series  of  floral  leaves, 
and  this  often  results  in  the  production  of  a  funnel- 
shaped  or  tubular  corolla  which  has  the  nectar  secreted 
at  the  bottom  of  the  flower,  where  it  can  be  reached  only 
by  insects  having  a  tongue  long  enough  to  probe  to  the 
bottom  of  the  corolla.  Much  less  frequently  this  tubu- 
lar form  of  the  flower  is  due  to  the  cohesion  of  the 
sepals  alone,  the  petals  remaining  quite  distinct,  as  we 
see  in  some  of  the  pink  family,  e.g.  the  carnation  and 
catchfly.  A  study  of  such  tubular  flowers  shows  that 
they  are,  for  the  most  part,  pollinated  through  the  agency 
of  butterflies  and  moths,  although  some  smaller  insects 
may  visit  them  for  the  pollen. 

The  characteristic  odors  of  so  many  flowers  are  also 
lures  for  insects,  and  sometimes,  as  in  the  mignonette, 
this  is  the  only  means  of  attracting  attention,  as  the 
flowers  are  very  inconspicuous  in  color.  Many  white 
flowers  have  a  peculiarly  strong  scent,  which  is  usually 
most  marked  at  night ;  indeed,  some  flowers  are  odorous 
only  at  night.  An  examination  of  these  pale,  night- 
scented  flowers  soon  reveals  the  fact  that  they  are  espe- 
cially adapted  to  attract  night-flying  insects,  the  white 
or  pale  yellow  color,  and  strong  odor,  making  them  more 
readily  found  in  the  twilight,  or  even  after  it  is  quite 


.        ANIMALS   AND   PLANTS  249 

dark.  A  flowering  vine  of  white  honeysuckle,  or  a  bush 
of  the  pale  yellow  evening  primrose,  may  often  be  seen 
at  dusk  to  be  swarming  with  great  sphinx  moths,  which, 
poised  before  the  flowers  like  humming-birds,  probe  the 
deep,  narrow  trumpets  with  their  long  tongues.  Pass- 
ing from  flower  to  flower  in  their  search  for  honey, 
cross-pollination  is  almost  certain  to  be  effected. 

While  the  chief  agents  in  the  pollination  of  flowers 
are  insects,  especially  butterflies  and  bees,  other  ani- 
mals also  may  be  of  importance  in  this  connection.  It 
is  said  that  snails  have  been  observed  to  convey  pollen 
from  the  flowers  of  some  aroids,  but  next  to  insects  it 
is  birds  which  play  the  most  important  r61e,  especially 
the  peculiar  American  group  of  humming-birds,  which 
are  preeminently  flower  visitors.  Although  in  north- 
eastern America  there  is  but  a  single  humming-bird, 
the  little  ruby-throat,  several  of  the  native  flowers  seem 
to  have  adapted  themselves  especially  to  its  visits. 
Among  the  most  striking  of  these  humming-bird  flow- 
ers, are  the  coral  honeysuckle,  cardinal  flower,  trumpet- 
creeper,  crimson  balm  (Monarda),  and  wild  columbine. 
All  of  these  have  deep,  narrow  nectaries,  and  scarlet 
is  the  predominant  color.  Of  the  garden  flowers, 
which  are  especial  favorites  of  the  humming-birds, 
may  be  mentioned  the  various  species  of  Canna,  fuch- 
sia, nasturtium  (Tropoeolum),  and  the  scarlet  Mexi- 
can sage.  In  California  the  fuchsia-like  Zauschneria 
and  the  crimson-flowered  currant  (Ribes  speciosum),  as 
well  as  a  number  of  other  bright  red  flowers,  are  eagerly 
sought  by  the  native  humming-birds.  It  will  be  noted 
that  nearly  all  these  flowers  are  vivid  red,  a  color  which 
appears  to  be  especially  attractive  to  these  little  birds. 


250 


EVOLUTION   OF   PLANTS 


While  at  first  sight  it  would  seem  that  flowers  having 
stamens  and  pistil  together  would  usually  be  self -pol- 
linated, such  is  rarely  the  case,  at  least  in  showy  flowers. 
An  examination  of  these  reveals  many  effective  arrange- 
ments by  which  this  is  prevented  and  cross-fertilization 
made  necessary.  One  of  the  commonest  and  simplest 
means  is  the  maturing  of  the  stamens  and  pistil  at  dif- 
ferent times.  Usually  it  is  the  stamens  which  are  ripe 


FIG.  56  (Cross-fertilization).  —  A,  flower  of  Erodium,  one  of  the  Geranium 
family ;  the  flower  is  inconspicuous  and  capable  of  self-pollination  ;  B, 
stamens  and  carpels  of  Erodium ;  the  stigmas,  st,  are  mature  when  the 
pollen  is  shed ;  C,  young  flower  of  Pelargonium ;  the  pistil,  p,  is  im- 
mature ;  D,  the  pistil  of  an  older  flower  which  has  shed  the  anthers ; 
the  stigmatic  lobes,  st,  are  now  ready  for  pollination ;  E,  young  flower 
of  a  nasturtium  (Tropoeolum)  ;  three  of  the  stamens  are  shedding  their 
pollen  and  occupy  the  space  in  front  of  the  opening  of  the  spur ;  the 
other  stamens  are  still  closed,  and  with  the  immature  pistil,  st,  are 
bent  down  ;  F,  stamens  and  pistil  of  an  old  flower ;  the  stamens  have 
all  shed  their  pollen,  and  the  receptive  stigma,  st,  now  occupies  the 
position  in  front  of  the  opening  of  the  spur ;  G,  flower  of  broom 
(Sarothamnus)  ;  the  stamens  and  pistil  are  included  within  the  keel, 
k;  H,  a  flower  which  has  had  the  keel  forced  down,  liberating  the 
stamens  and  pistil. 

first    (proterandry),    but    proterogyry,    or    the    earlier 
maturing  of  the  pistil,  may  also  occur,  e.g,  the  common 


ANIMALS    AND   PLANTS  251 

plantain.  A  familiar  case  of  proterandiy  is  seen  in  the 
common  scarlet  geranium  and  other  species  of  Pelargo- 
nium (Fig.  56,  C,  D).  The  stamens  are  ripe  at  the 
time  the  flower  first  opens,  and  the  pollen  is  shed  almost 
at  once,  but  at  this  time  the  stigma  is  quite  closed,  and 
the  stigmatic  surface  cannot  receive  pollen.  In  the 
older  flowers,  after  the  pollen  is  shed,  the  five  lobes  of 
the  stigma  spread  out  widely  and  the  stigmatic  sur- 
faces are  exposed,  but  pollen  must  necessarily  be  brought 
from  a  younger  flower. 

A  similar  but  more  complicated  arrangement  is 
seen  in  the  nasturtium  (Fig.  56,  E,  F).  Like  Pelargo- 
nium there  are  seven  stamens,  which  discharge  their 
pollen  before  the  stigma  is  in  a  receptive  condition. 
The  flower  here  is  strongly  zygomorphic,  and  the  two 
lower  petals  are  so  placed  as  to  form  a  resting-place  for 
the  bumblebees  which  are  the  commonest  visitors  to 
the  flowers.  In  addition  to  this,  two  of  the  sepals  are 
joined  to  form  the  long  spur  or  nectary  which  the  bee 
must  probe  for  the  honey  contained  at  its  apex.  The 
seven  stamens  in  a  young  flower  are  all  bent  downward 
(Fig.  56,  E),  but  as  they  mature  they  rise,  one  by  one, 
so  that  the  open  anther  stands  directly  before  the  open- 
ing of  the  spur,  and  any  insect  seeking  for  honey  must 
infallibly  rub  off  some  of  the  pollen.  After  all  the 
stamens  have  discharged  their  pollen  they  turn  down 
again,  and  their  place  is  taken  by  the  pistil,  which  has 
in  the  meantime  elongated,  and  the  three  stigmatic 
lobes  have  opened  and  are  ready  to  receive  the  pollen. 
The  open  stigma  now  occupies  exactly  the  same  position 
as  did  the  open  anthers,  and  any  insect  which  has 
visited  a  younger  flower  is  sure  to  deposit  upon  the 


252  EVOLUTION   OF   PLANTS 

stigma  the  pollen  brought  from  it.  In  both  Pelargo- 
nium and  Tropceolum  self-fertilization  is  impossible. 

The  pea  family  offers  many  striking  examples  of 
flowers  which  are  entirely  dependent  upon  insects  for 
pollination.  The  peculiar  butterfly-shaped  flowers  of 
most  of  these  have  the  stamens  and  pistil  enclosed  in 
the  "keel "  formed  by  the  union  of  the  two  lower  petals 
(Fig.  56,  G,  &).  The  pollen  is  discharged  and  forms 
a  loose,  powdery  mass  within  the  keel,  but  cannot  reach 
the  stigma  owing  to  the  presence  of  a  brush  of  hairs 
between  it  and  the  stigma.  If  a  bee  alights  upon  the 
flower,  in  searching  for  the  honey  the  sides  of  the  keel 
are  forced  downward,  and  the  apex  of  the  pistil  is  ex- 
posed, usually  springing  out  with  some  force  and  brush- 
ing out  the  pollen,  which  is  thus  dusted  upon  the  visitor, 
which  carries  it  to  the  next  flower,  where  it  is  deposited 
upon  the  stigma.  In  some  Leguminosce,  like  the  species 
of  broom  (Sarothamnus)  (Fig.  56,  G,  H),  the  stamens 
and  pistil  are  set  free  with  a  good  deal  of  violence,  and 
there  is  a  small  explosion  when  the  keel  is  depressed, 
and  the  pollen  is  ejected  with  considerable  force. 

Certain  parts  of  the  flower  may  be  sensitive  to  touch, 
and  this  is  almost  always  connected  with  pollination. 
Thus  in  the  common  barberry  the  stamens  are  extremely 
sensitive  and  on  being  touched  near  the  base,  as  happens 
when  an  insect  is  seeking  for  nectar,  they  spring  in- 
ward with  a  quick  motion  and  deposit  the  pollen  upon 
the  visitor.  The  trumpet-creeper  (Tecoma),  and  other 
related  plants  have  the  stigma  sensitive,  the  two  lobes 
of  which  it  is  composed  closing  slowly  after  they  are 
touched.  This  is  possibly  a  provision  for  holding  the 
pollen-grains  deposited  upon  it,  and  perhaps  hasten- 


ANIMALS   AND   PLANTS  253 

ing  the  germination.  The  mountain  laurel  (Kalmia) 
of  eastern  America  has  the  stamens  in  the  freshly 
opened  flower  bent  outward,  and  the  anther  fitted  into 
a  little  pocket  from  which  it  is  set  free  by  an  insect  visit- 
ing the  flower;  the  suddenly  released  stamens  spring 
inward  much  as  in  the  barberry  and  scatter  their  pollen 
in  the  same  way. 

Among  the  sympetalous  Dicotyledons  the  devices  for 
effecting  cross-pollination  are  often  exceedingly  perfect. 
Most  of  these  have  tubular  and  often  two-lipped  flowers 
which  are  very  generally  incapable  of  self-fertilization. 
The  labiate  flowers  are  usually  horizontal  or  pendulous, 
and  often  adapted  to  special  insects.  Thus  the  common 
foxglove  (Digitalis)  is  mainly  visited  by  large  bees, 
which  creep  into  the  bell-shaped  corolla,  where  the  back 
comes  in  contact  with  the  open  anthers  which  lie  against 
the  upper  part  of  the  corolla.  Here  the  stamens  mature 
first,  so  that  ordinarily  the  pistil  is  pollinated  by  pollen 
from  a  younger  flower,  but  it  is  said  that  in  case  insects 
are  prevented  from  visiting  the  flower,  self-fertilization 
is  possible. 

In  various  Labiatse,  or  Mints,  e.g.  Lamium,  Salvia 
(Fig.  57,  A,  B),  the  arrangements  for  cross-fertilization 
are  very  complete,  and  probably  in  both  of  these  genera 
self-fertilization  is  impossible.  In  the  former,  while 
stamens  and  pistil  mature  about  the  same  time,  the 
stigma  hangs  below  the  stamens,  and  its  receptive  sur- 
face is  turned  away  from  them  so  that  no  pollen  can 
fall  on  it  from  above,  and  a  bee  entering  the  flower, 
with  pollen  taken  from  another  one,  will  touch  the 
stigma  and  deposit  the  pollen  upon  it,  before  it  comes 
in  contact  with  the  stamens.  In  the  various  species 


254 


EVOLUTION  OF   PLANTS 


of  sage  (Salvia)  the  flower  is  shaped  much  as  in  La- 
ra ium,  but  the  stamens  are  reduced  to  two,  and  the 
pistil  does  not  mature  until  after  the  pollen  is  shed,  so 
that  self-pollination  is  quite  impossible.  The  anther 
is  very  peculiar  in  form  and  balanced  upon  the  short 
filament,  so  that  an  insect  striking  against  the  lower 
end  of  the  elongated  anther  pushes  the  upper  end, 
with  the  pollen,  down  upon  its  back  (Fig.  57,  A).  At 


2DC 


FIG.  57  (Cross-fertilization).  —  A,  a  flower  of  a  sage,  Salvia  pratensis, 
showing  the  way  in  which  a  bee,  visiting  the  flower,  forces  down  the 
stamens  so  that  the  anthers,  an,  strike  its  body;  the  stigma,  st,  is  not 
in  a  position  to  be  hit  by  the  insect;  B.  an  older  flower  of  the  same; 
the  style  has  elongated  so  that  the  stigma  will  be  pollinated  by  a  bee 
which  has  already  visited  another  flower ;  the  position  of  the  undis- 
turbed stamens  is  indicated  by  the  dotted  lines :  C,  flower  of  a  milk- 
weed (Asclepias)  ;  p,  the  cleft  through  which  the  pollinia  are  extracted ; 
D,  median  section  of  the  milkweed  flower;  st,  stigma;  p,  pollen-mass, 
or  pollinium :  an,  the  base  of  the  stamen;  E,  a  pair  of  pollinia  with- 
drawn from  the  anther;  F, the  flower  of  an  orchid  (Orchis spectabilis) ; 
the  upper  perianth  leaves  are  bent  back  to  expose  the  column,  or  gyno- 
stemium,  gy ;  I,  the  lip  prolonged  backward  into  the  spur,  sp :  o,  the 
ovary;  G,  the  column  of  F,  seen  from  in  front;  an,  the  anther,  con- 
sisting of  the  two  receptacles,  each  containing  a  pollinium  terminating 
in  the  disk,  d;  st,  one  of  the  stigmatic  surfaces  on  each  side  of  the 
opening  of  the  spur ;  H-J,  the  pollinia  removed  from  the  anther,  show- 
ing the  change  of  position  on  exposure  to  the  air.  (Figs.  A,  B  after 
Noll.) 


ANIMALS   AND   PLANTS  255 

this  time  the  style  is  still  short,  but  in  older  flowers 
(B)  the  style  elongates  and  bends  down,  so  that  the 
receptive  stigma  (s£)  occupies  the  same  position  as 
did  the  anther  in  the  younger  flower;  and  when  a  bee 
enters,  with  its  back  dusted  with  pollen,  some  of  this 
is  certain  to  adhere  to  the  stigma. 

The  milkweed  family  exhibits  another  peculiar  method 
of  cross-fertilization.  In  the  common  milkweed  (As- 
clepias)  the  very  peculiar  flowers  (Fig.  56,  C)  are 
characterized  by  having  the  pollen  in  little  packets 
(pollinia)  (Fig.  57,  E),  which  are  contained  in  closed 
receptacles  and  can  be  dislodged  only  through  insect 
agency  and  by  using  considerable  force. .  Indeed,  it  is 
not  unusual  for  the  butterflies,  which  are  the  common 
agents  in  pollination  here,  to  have  their  tongues  or  legs 
caught  so  firmly  in  the  clefts  through  which  the  pollen- 
masses  are  ordinarily  extracted,  that  they  are  held  fast, 
and  perish.  The  pollinia  are  provided  with  adhesive 
disks  by  which  they  become  firmly  attached  to  the  head 
or  legs  of  the  insect,  and  are  carried  thus  to  other 
flowers. 

Most  remarkable  of  all  flowers,  however,  are  some 
of  the  orchids,  among  which,  perhaps,  are  found  the 
most  specialized  of  all  floral  structures.  The  flowers  of 
some  orchids  are  of  great  size  and  wonderful  beauty, 
and  some  of  them  exhibit  most  marvellous  contrivances 
for  insuring  cross-fertilization.  One  of  the  simpler 
types  is  shown  in  the  figure  (Fig.  57,  F-J),  and  will 
illustrate  the  character  of  these  mechanical  arrange- 
ments. As  in  all  orchids,  one  of  the  petals  is  modified 
into  the  "  lip  "  (Z),  which  is  prolonged  backward  into 
a  long  hollow  spur  (sp),  which  forms  the  nectary. 


256  EVOLUTION   OF  PLANTS 

The  stamens  (here  reduced  to  a  single  one)  and 
pistil  are  grown  together  into  a  "  column  "  or  "  g}^no- 
stemium."  Like  the  milkweed,  this  orchid  has  the 
pollen -spores  in  two  pollinia,  club-shaped  masses  con- 
verging toward  the  base,  where  each  terminates  in 
a  sticky  disk  covered  over  with  a  delicate  membrane 
just  above  the  opening  of  the  spur  (G,  d).  Each  pol- 
linium  lies  in  a  little  pocket  from  which  it  can  be  dis- 
lodged only  through  some  external  agency.  An  insect 
alighting  upon  the  lip  and  probing  the  spur  for  nectar, 
must  hit  against  the  membrane  which  covers  the  base 
of  the  pollinia,  and  this  is  ruptured,  and  the  adhesive 
disks  are  thus  brought  into  contact  with  the  head  or 
tongue  of  the  insect,  to  which  they  become  firmly  at- 
tached by  the  "setting"  of  the  cement-like  substance 
composing  the  disk.  As  the  insect  backs  out  of  the 
flower,  the  two  pollinia  are  dragged  out  of  their  recep- 
tacles and  carried  away.  The  action  of  the, insect  is 
easily  imitated  by  inserting  into  the  flower  a  slender 
stalk  of  grass,  or  the  fine  point  of  a  pencil,  which  on 
being  withdrawn  will  drag  away  the  pollinia.  The 
latter  at  first  stand  nearly  vertical  and  diverge  widely 
(H)  ;  but  very  quickly  they  change  position,  bending 
downward  and  forward  until  they  lie  nearly  parallel 
and  point  almost  directly  forward  (I,  J).  Thrusting 
the  pencil-point  with  the  pollinia  in  this  position,  into 
another  flower,  it  will  be  found  that  the  pollinia  come 
into  immediate  contact  with  the  two  stigmatic  surfaces 
on  either  side  of  the  opening  of  the  spur  (Fig.  57,  G,  s£), 
lower  down  than  the  anther. 

Many  other  even  more  remarkable  instances    might 
be  cited,  but  space  forbids  a  further  discussion  of  this 


ANIMALS   AND  PLANTS  257 

most  interesting  topic  here.  The  works  of  Darwin, 
Miiller,  and  others  may  be  consulted  by  those  who 
desire  to  become  further  acquainted  with  the  really 
astonishing  contrivances  found  among  the  orchids. 

In  most  instances,  flowers  are  visited  by  insects  either 
for  nectar  or  for  pollen,  but  there  are  some  exceptions  to 
this.  One  of  the  most  remarkable  cases  is  that  of  vari- 
ous species  of  Yucca,  which  are  most  abundant  in  the 
arid  regions  of  southwestern  America.  In  the  species 
which  have  been  investigated,  the  agent  in  pollination 
is  a  small  moth  of  the  genus  Pronuba,  whose  larvae  feed 
upon  the  young  seeds.  The  moth  deposits  its  eggs  in 
the  young  ovary  of  the  flower,  and  then  deliberately 
crowds  a  mass  of  pollen  into  the  canal  of  the  stigma, 
thus  insuring  the  fertilization  of  the  ovules.  The  larvae 
hatch  and  feed  upon  the  growing  seeds,  some  of  which, 
however,  are  left  uninjured,  and  ripen  after  the  rest 
have  been  eaten  by  the  larvae. 

A  very  remarkable  group  of  plants  are  those  known 
as  "  carnivorous  "  or  "  insectivorous  "  plants,  which  in- 
stead of  being  eaten  by  animals,  themselves  capture 
and  devour  insects  and  other  small  animals.  In  some 
instances,  like  the  common  sundew  (Drosera)  (Fig.  58, 
C,  D)  and  Venus's  flytrap  (Dionsea),  the  insects  are  cap- 
tured alive,  and  actually  digested.  In  these  plants  the 
leaves  are  sensitive,  and  an  insect  alighting  upon  a  leaf 
is  either  held  fast  by  means  of  a  sticky  secretion,  which 
increases  in  amount  as  the  leaf  is  stimulated  by  the 
movements  of  the  insect,  and  then  slowly  folds  up  about 
it ;  or,  in  Dionaea,  the  blade  of  the  leaf  is  arranged  much 
like  the  jaws  of  a  spring-trap,  and  closes  up  quickly 
when  the  insect  touches  certain  sensitive  hairs  upon 


258 


EVOLUTION   OF   PLANTS 


its  upper  surface.  The  peculiar  digestive  fluid  which  is 
present  in  both  cases  is  probably  a  direct  product  of  the 
plant  itself,  although  it  has  been  claimed  that  it  is  due 
to  the  presence  of  certain  bacteria,  which  are  present  in 
large  numbers.  Whether  the  digestive  process  is  due 
to  the  secretions  of  the  plant  itself,  or  to  the  activity 
of  the  bacteria,  the  products  of  digestion  undoubtedly 

serve  to  supply  the  plant 
with  nitrogenous  food. 

The  pitcher-plants, 
Nepenthes  (Fig.  58,  B), 
Sarracenia  (Fig.  58,  A), 
and  Darlingtonia,  are 
also  striking  examples  of 
these  carnivorous  plants. 
All  of  these  have  the 
leaves  modified  into 
pitcher  -shaped  „  recepta- 
cles, into  which  insects 
are  lured  by  the  bright 
colors  of  the  leaves  as 
well  as  an  abundant  se- 


FIG.  58  (Carnivorous  plants).  —  A, 
leaf  of  the  common  pitcher-plant 
(Sarracenia  purpurea)  ;  B,  pitcher 
of  a  tropical  pitcher-plant  (Nepen- 
thes) borne  upon  the  end  of  a  ten- 
dril, the  opening  protected  by  a 
lid;  C,  leaf  of  a  sundew  (Drosera 
lonyifolia)  ;  D,  leaf  of  Drosera 
which  has  captured  a  mosquito, 
showing  the  way  in  which  the  ten- 
tacles, ten ,  clasp  the  insect ;  E,  part 
of  a  leaf  of  bladder-weed  (Utricu- 
laria),  with  the  bladder-like  trap, 
v;  F,  a  single  vesicle  of  Utricu- 
laria  more  enlarged.  (Figs.  A,  B 
after  Goebel.) 

insect  which   has   fallen   in 


cretion  of  a  honey-like 
substance.  In  most  of 
them  the  upper  part  of 
the  interior  of  the  pitcher 
is  lined  with  stiff,  down- 
ward pointing  hairs,  be- 
low which  the  wall  is 
very  smooth,  so  that  an 
cannot  escape.  A  fluid 


is   secreted   by    the    pitcher,    which    partially   fills   it, 


ANIMALS  AND   PLANTS  259 

and  acts,  to  some  extent  at  least,  as  a  digestive 
fluid. 

The  bladder-weeds  (Utricularia)  (Fig.  58,  E,  F)  and 
the  butterworts  (Pinguicula)  are  also  well-known  ex- 
amples of  carnivorous  plants.  The  former  are  aquatics, 
whose  finely  dissected  leaves  are  provided  with  little 
bladder-like  vesicles,  which  form  perfect  traps  for  small 
Crustacea,  and,  it  is  said,  in  some  cases  for  young  fish. 
In  all  of  these  carnivorous  plants,  this  peculiar  habit  is 
evidently  a  provision  for  providing  them  with  nitrogen. 
They  are  always  either  bog-plants,  or  actually  aquatic, 
and  the  roots  are  poorly  developed  or  quite  wanting,  so 
that  they  are  inadequate  to  provide  the  plants  with  the 
amount  of  nitrogen  necessary  for  their  growth,  especially 
as  the  medium  in  which  they  grow  is  apt  to  be  deficient 
in  nitrogenous  matter. 

We  find,  among  the  higher  plants  especially,  many 
devices  for  protecting  them  against  the  attacks  of  ani- 
mals which  seek  them  for  food.  These  protective  de- 
vices are  of  very  different  character  in  different  forms. 
Thus  many  plants,  such  as  the  majority  of  perennial 
grasses,  have  creeping  underground  stems  which  send 
up  leaves  at  a  great  many  points,  and  these  leaves  are 
capable  of  continued  basal  growth,  and  may  be  eaten 
down  close  to  the  ground,  growing  up  again  promptly, 
so  that  the  destruction  of  the  plant  is  almost  impossi- 
ble. It  is  this  tenacity  of  life  which  makes  many  of 
the  grasses  such  troublesome  weeds.  It  is  extremely 
probable  that  the  development  of  acrid  or  poisonous 
substances,  or  ill-scented  essential  oils,  in  the  leaves  of 
many  plants,  is  primarily  protective,  and  makes  the 
plants  offensive  to  animals.  That  these  secretions  do 


260  EVOLUTION   OF  PLANTS 

not  render  the  plants  entirely  immune,  is  shown,  how- 
ever, by  the  attacks  of  certain  animals,  especially  insects, 
which  have  apparently  adapted  themselves  to  these 
peculiar  conditions.  Nevertheless,  there  is  no  question 
that  such  plants  suffer  very  much  less  from  animals 
than  they  would  if  these  means  of  protection  were 
absent.  It  has  been  thought  that  the  sharp  needle-like 
crystals  or  rhaphides,  which  occur  so  abundantly  in 
many  Monocotyledons,  may  deter  animals  from  eating 
them,  as  many  of  them,  especially  the  aroids,  have  an 
excessively  acrid  taste,  which  is  supposed  to  be  due 
to  the  mechanical  irritation  produced  by  these  sharp 
crystals. 

The  presence  of  spines,  thorns,  and  prickles,  as  well 
as  rough  hairs  upon  the  stems  and  leaves,  is  doubtless 
mainly  protective.  They  are  usually  most  noticeable 
in  plants  of  dry  regions,  where  the  scanty  vegetation  is 
peculiarly  exposed  to  the  attacks  of  herbivorous  .animals. 
The  cacti  are  very  perfect  instances  of  this  peculiarity. 
The  terribly  sharp  thorns  of  these  plants  render  them 
perfectly  safe  against  the  attacks  of  hungry  animals, 
which  eat  them  greedily  if  care  is  taken  to  remove 
their  thorny  armor.  Where  desert  plants  are  not 
thorny,  they  are  usually  ill-scented  and  thus  distaste- 
ful to  herbivorous  animals. 

But  one  more  of  the  most  remarkable  cases  of  recip- 
rocal relations  between  plants  and  animals  will  be  cited, 
namely,  the  peculiar  arrangement  known  as  myrmecoph- 
ily,  where  ants  inhabit  certain  parts  of  trees,  to  which, 
in  return  for  shelter,  and  sometimes  food  in  the  form 
of  honey-like  secretions  or  peculiar  albuminous  fatty 
bodies,  they  protect  the  plants  from  the  ravages  of  other 


ANIMALS   AND  PLANTS  261 

insects  or  larger  animals.  One  of  the  best  known 
cases  of  this  kind  is  that  of  the  tropical  American 
genus  Cecropia,  trees  with  large  palmate  leaves,  some 
species  of  which  have  the  stems  enlarged  and  hollow, 
serving  as  the  abode  of  certain  ants  which  keep  away 
the  leaf-cutting  ants,  which  otherwise  do  great  damage 
to  the  tree  by  eating  the  foliage.  The  leaf-cutting 
ants,  in  their  turn,  utilize  the  leaves  for  the  formation  of 
miniature  hotbeds  upon  which  they  are  said  actually  to 
cultivate  a  certain  fungus  which  they  use  as  food.  Some 
species  of  Acacia  develop  large  hollow  thorns,  which 
serve  as  shelters  for  ants  which  are  also  furnished  with 
food-bodies  like  those  of  Cecropia,  and  in  return  protect 
their  host  against  its  animal  foes.  There  are  a  number 
of  other  more  or  less  well-authenticated  cases  of  myr- 
mecophily. 


CHAPTER   XIV 

INFLUENCE   OF  ENVIRONMENT 

THE  conditions  for  normal  plant-growth  are  light, 
heat,  moisture,  and  certain  food  constituents,  including 
carbon  dioxide,  oxygen,  and  some  nitrogen  compounds. 
As  these  conditions  necessarily  are  not  constant  in  all 
cases,  we  find,  as  might  be  expected,  a  corresponding 
variation  in  different  plants  by  which  they  have  accom- 
modated themselves  to  these  varying  conditions. 

Most  of  the  lower  green  plants  are  aquatic  and  all 
their  cells  are  equally  exposed  to  the  medium  in  which 
they  live.  These  plants  being  unicellular  or  composed 
of  simple  cell-aggregates  made  up  of  similar  cells, 
each  cell  is  capable  of  performing  the  different  plant 
functions,  which  in  more  highly  specialized  plants  are 
relegated  to  special  cells.  Each  cell  of  these  simple 
plants  absorbs  water  containing  the  necessary  food 
constituents  in  solution,  and  as  all  the  cells  contain 
chlorophyll,  all  are  able  to  decompose  the  carbon  di- 
oxide dissolved  in  the  imbibed  water.  The  free  oxygen 
needed  by  the  plant  is  also  taken  in  with  the  water. 
Associated  with  the  aquatic  habit  of  these  plants  is  the 
power  of  active  locomotion  so  often  seen  in  their  repro- 
ductive cells. 

The  marine  forms  allied  to  these  simple  algse  have 
become  much  changed  in  some  respects,  and  notably  in 

262 


INFLUENCE   OF   ENVIRONMENT  263 

the  color  of  those  which  grow  in  deeper  water.  Most 
of  the  red  algse  belong  to  this  category,  and  the  devel- 
opment of  a  special  red  pigment  allied  to  chlorophyll 
seems  to  be  a  provision  for  increasing  the  absorp- 
tion of  certain  of  the  light  rays  which  pass  through 
the  water,  and  is  doubtless  concerned  in  some  way, 
more  or  less  directly,  with  the  question  of  carbon 
assimilation.  The  brown  and  yellow  pigments  of  the 
Phaeophyceae  are  probably  purely  protective,  acting  as 
screens  for  the  chlorophyll  when  the  plants  are  exposed 
at  low  tide. 

Living  in  a  medium  which  is  of  approximately  equal 
density  with  the  plant  itself,  most  algae  develop  no 
supporting  or  mechanical  tissues,  being  buoyed  up  by 
the  water  in  which  they  float;  such  forms  on  being 
removed  from  the  water  collapse  completely.  They 
also  have  no  protection  against  the  loss  of  water  by 
evaporation,  and  this,  when  they  are  exposed  to  the  air, 
is  very  rapid  and  complete.  Where,  however,  water 
plants  are  exposed  to  the  beating  of  the  surf,  as  is  the 
case  in  many  of  the  large  kelps  and  some  red  algse,  like 
the  common  Irish  moss  (  Ohondrus  crispus),  the  cell-walls 
of  the  outer  tissues  become  firm  and  cartilaginous  in 
consistence,  so  that  the  plant  is  very  tough  and  flexible 
and  can  endure  the  buffeting  of  the  heavy  surf  without 
injury,  and  the  mucilaginous  nature  of  their  inner 
tissues  prevents  too  rapid  loss  of  water  when  they  are 
exposed  to  the  air.  These  surf  plants  develop  root- 
like  holdfasts,  which  anchor  them  firmly,  so  that  they 
can  be  torn  away  from  their  moorings  only  by  the  exer- 
cise of  considerable  force.  In  the  largest  of  these  kelps, 
as  we  have  seen,  the  leaves  are  provided  with  floats 


264  EVOLUTION  OF  PLANTS 

which  bring  them  near  the  surface  of  the  water  where 
they  may  be  exposed  to  the  light. 

One  of  the  most  important  differences  between  fresh- 
water and  marine  algae,  resulting  from  the  nature  of 
their  environment,  is  the  different  character  of  some  of 
the  reproductive  parts.  Owing  to  the  constant  level 
of  the  ocean,  aside  from  the  periodic  fluctuation  of  the 
tides,  marine  plants  are  never  exposed  to  the  complete 
desiccation  to  which  nearly  all  fresh-water  plants  are 
at  times  liable ;  nor  is  there  nearly  so  much  difference 
of  temperature  in  the  water  at  different  seasons,  as  in 
the  shallower  and  usually  variable  body  of  water  in 
most  lakes  and  rivers.  We  find,  therefore,  that  the 
marine  algae  do  not  develop  resting-spores  except  in 
rare  instances,  but  the  spores  are  thin-walled  or  naked 
cells  which  germinate  as  soon  as  they  are  mature. 
Where  the  plants  show  a  definite  periodicity  in  their 
growth,  as  not  infrequently  occurs  in  the  colder 
northern  waters,  the  plant  is  usually  perennial  by 
means  of  a  sort  of  root-stock,  or  rhizome,  from  which 
the  annual  shoots  are  produced. 

Most  fresh-water  algse,  however,  are  plants  of  very 
limited  growth,  and  are  usually  destroyed  either  by 
freezing  or  drying  up  at  the  end  of  their  growing 
season.  In  the  great  majority  of  these  are  produced 
special  reproductive  bodies,  usually  resting-spores, 
which  are  capable  of  resisting  extremes  of  tempera- 
ture and  dryness  which  would  quickly  destroy  the 
actively  vegetating  plant.  These  resting-spores  are 
commonly  the  result  of  fertilization,  but  not  infre- 
quently they  may  form  non-sexually,  as  we  find  in 
various  of  the  fission  algae,  like  Nostoc  or  Anabaena. 


R  A 

VM 

INFLUENCE   OF  ENVIROl(MgM?i  ^  26; 


These  resting-spores  are  usually  pix)ducefl?~a&  tirtEdo- 
gonium  or  Spirogyra,  at  the  end  of  the  plant's  exist- 
ence, after  which  the  vegetative  cells  die,  leaving  the 
thick-walled  resting-spores  to  carry  the  plant  over  to 
the  next  growing  season. 

These  fresh-water  plants  are,  as  a  rule,  far  more 
resistant  to  changes  of  temperature  than  their  marine 
relatives,  which  frequently  are  killed  very  quickly  by 
a  slight  rise  in  temperature  in  the  water,  this  being 
especially  marked  in  the  deep-water  red  algse,  which 
are  only  adapted  to  an  environment  where  the  tem- 
perature remains  almost  constant  and  where  they  are 
protected  from  strong  illumination.  This  great  sensi- 
tiveness makes  the  cultivation  in  aquaria  of  most 
marine  algse  exceedingly  difficult. 

The  origin  of  the  first  terrestrial  plants  was  due, 
probably,  to  the  survival  of  some  algal  form,  which, 
instead  of  dying  as  soon  as  the  spores  were  ripe,  con- 
tinued to  vegetate  upon  the  mud  after  the  subsidence 
of  the  water,  as  is  still  the  case  in  a  few  algse.  Some 
of  the  lower  liverworts,  which  probably  resemble  more 
nearly  than  any  existing  forms  these  primitive  terres- 
trial plants,  still  show  this  amphibious  habit,  floating 
in  the  water  during  most  of  their  life,  but  finally  com- 
pleting their  development  upon  the  mud  left  by  the 
evaporation  of  the  water.  The  capability  of  growing 
with  a  diminished  water  supply  is  an  obvious  advan- 
tage, and  this  is  shown  by  the  rapid  evolution  of  these 
land  plants  which  has  resulted  in  an  immense  number 
of  most  diverse  types. 

The  mosses,  which  are  doubtless  descended  from 
aquatic  ancestors,  in  adapting  themselves  to  their  new 


266  EVOLUTION   OF   PLANTS 

terrestrial  environment  have  become  greatly  modified. 
Thus  there  are  developed  various  provisions  against 
injury  from  loss  of  water,  either  by  the  plants  as  a 
whole  acquiring  the  power  of  becoming  completely 
dried  up  without  being  killed,  or  the  outer  tissues  of 
the  plant  becoming  more  or  less  impervious  to  water ;  or 
the  more  delicate  portions  may  be  protected  in  various 
ways  from  the  injurious  effects  of  drouth.  The  tissues 
are  always  firmer  than  those  of  water  plants,  as  the 
plant  no  longer  is  supported  by  the  medium  in  which 
it  is  growing,  but  must  depend  upon  the  rigidity  of  its 
own  tissues. 

The  spores  in  the  mosses  and  all  the  higher  plants 
have  lost  the  power  of  locomotion  possessed  by  the 
zoospores  of  the  aquatic  algae,  and  this  loss  of  motion, 
as  well  as  the  thick  walls  with  which  they  are  fur- 
nished, are  adaptations  to  the  changed  environment, 
where  the  spores  depend  for  their  distribution,  not 
upon  water,  but  upon  air  currents.  It  is  interesting  to 
recall  that  even  in  these  terrestrial  plants  there  is  a 
reversion  to  the  primitive  aquatic  condition  when  fer- 
tilization is  effected. 

The  abandonment  of  the  aquatic  habit  in  the  higher 
plants  is  associated  with  marked  increase  in  the  impor- 
tance of  the  sporophyte,  or  non-sexual  spore-bearing 
generation.  This  first  results  in  the  very  marked 
alternation  of  generations  in  the  Archegoniates,  — 
mosses  and  ferns, —  and  finally  has  produced  the  seed 
plants,  where  the  gametophyte  is  greatly  reduced  and 
is  never  capable  of  independent  existence.  The  inde- 
pendence of  the  sporophyte,  first  found  in  the  ferns,  is 
associated  with  the  development  of  special  organs,  stem, 


INFLUENCE   OF   ENVIRONMENT  267 

leaves,  and  roots,  and  as  soon  as  this  stage  was  fully 
evolved,  an  entirely  new  type  of  plant  structure  had 
come  into  existence,  which  was  destined  to  become  the 
predominant  type  of  the  future,  finally  culminating  in 
the  great  group  of  seed-bearing  plants. 

It  is  among  the  latter  that  there  are  found  the  most 
remarkable  and  perfect  adaptations  to  special  condi- 
tions. Being  mostly  terrestrial  plants,  they  show, 
when  compared  with  the  lower  plants,  which  are  for 
the  most  part  aquatic,  a  much  greater  development  of 
mechanical  tissues,  by  which  the  stem  and  leaves  may 
be  supported.  The  most  highly  developed  of  these 
mechanical  tissues  is  the  wood  of  the  vascular  bundles, 
which  forms  the  great  part  of  the  skeleton  of  the  stems 
of  trees  and  shrubs,  and  also  the  framework  of  the 
leaves.  The  vascular  bundles  are  first  met  with  in  the 
ferns,  but  occur  in  all  the  higher  plants.  In  some 
vascular  plants,  like  most  Monocotyledons,  the  wood 
is  poorly  developed  and  of  little  use  as  a  support- 
ing tissue,  and  in  these,  as  well  as  in  many  herba- 
ceous Dicotyledons,  the  mechanical  tissues  of  the  stem 
belong  principally  to  the  outer  part  of  the  ground-tissue, 
especially  the  layers  of  cells  just  below  the  epidermis. 
These  are  frequently  provided  with  thickened  walls  so 
that  the  mechanical  tissue  forms  a  cylinder  just  below 
the  epidermis,  which  is  itself  often  furnished  with 
thickened  cell-walls. 

The  woody  tissue  reaches  its  greatest  development 
in  the  stems  of  those  Conifers  and  Dicotyledons  which 
increase  in  diameter  from  year  to  }^ear  owing  to  the 
presence  of  the  so-called  "open"  vascular  bundles,  i.e. 
those  in  which  there  is  a  permanent  zone  of  growing 


268  EVOLUTION  OF   PLANTS 

tissue  or  cambium.  It  is  interesting  to  note  that  this 
peculiar  type  of  growth  has  developed  quite  indepen- 
dently in  several  widely  separated  groups  of  plants, 
apparently  in  response  to  similar  conditions  of  growth. 
Not  only  do  we  find  it  in  the  unrelated  Conifers  and 
Dicotyledons,  but  also  in  certain  Pteridophytes,  both 
fossil  and  recent.  In  the  Monocotyledons,  when  the 
plant  reaches  tree-like  proportions,  the  rigidity  of 
the  trunk  is  brought  about  in  part  by  a  large  develop- 
ment of  strengthening  tissue  in  the  outer  part  or  cor- 
tical region  of  the  stem,  and  partly  by  the  presence 
of  a  great  many  separate  vascular  bundles,  each  of 
which  is  usually  surrounded  by  a  sheath  of  supporting 
cells. 

The  great  mass  of  stems  and  foliage  in  the  larger 
flowering  plants  necessitates  a  very  perfect  system  of 
roots,  both  for  anchoring  the  plant  firmly  in  the  earth, 
and  for  supplying  it  with  water  and  various  food 
elements.  In  Gymnosperms  and  Dicotyledons,  which 
have  woody  trunks,  there  is  very  often  a  main  or 
tap-root  which  is  a  direct  continuation  of  the  stem, 
and,  like  it,  continues  to  increase  in  diameter 
through  the  permanent  growth  by  its  vascular  bundles. 
In  the  comparatively  small  number  of  arborescent 
Monocotyledons,  like  the  palms  and  screw-pines,  the 
necessary  support  is  given  by  a  great  many  stout  ad- 
ventitious or  secondary  roots,  which,  however,  are 
usually  incapable  of  secondary  growth  in  thickness. 
The  screw-pines  {Pandanus)  are  especially  remarkable 
in  the  development  of  these  roots  from  points  far  above 
the  ground,  and  the  trunk  is  often  supported  by  a  great 
number  of  these,  which  form  a  conical  mass  of  buttress- 


INFLUENCE   OF  ENVIRONMENT  269 

like  supports.     The  base  of  the  stem  of  the  common 
Indian  corn  shows  the  same  thing  on  a  small  scale. 

While  the  mechanical  tissues  are  of  course  best  de- 
veloped in  the  stem  and  roots,  the  leaves,  too,  as  we 
have  intimated,  may  also  be  used  to  illustrate  the  for- 
mation of  such  tissues,  and  in  Dicotyledons  especially, 
the  framework  for  supporting  in  the  air  the  widely 
expanded  leaves  is  very  perfect.  In  Monocotyledons 
the  leaves  stand,  as  a  rule,  more  nearly  upright,  and 
are  commonly  linear  in  form  with  a  much  less  perfect 
skeleton  than  that  found  in  most  Dicotyledons. 

AQUATIC  PLANTS 

The  contrast  between  the  development  of  the  me- 
chanical tissues  in  closely  related  forms  of  aquatic  and 
terrestrial  plants  is  very  instructive.  Aquatic  plants 
are  of  two  kinds,  those  which  are  entirely  submerged, 
and  those  with  floating  leaves.  These  two  differ  greatly 
in  the-  character  of  the  leaves,  which  in  the  former  class 
are  either  narrowly  linear  or  very  much  dissected  so  as  to 
expose  a  maximum  surface  for  the  absorption  of  carbon 
dioxide.  This  is  taken  in  directly  by  the  superficial 
cells  which,  unlike  those  of  most  land  plants,  contain 
chlorophyll  and  have  thin  outer  walls  which  allow  of 
free  interchange  of  fluids  and  gases.  No  cuticle  is 
developed  upon  the  epidermis,  and  the  result  of  this  is 
seen  when  these  submersed  aquatics  are  exposed  to  the 
air,  where  they  wither  up  almost  instantly,  owing  to 
the  rapid  evaporation  of  the  water  from  them.  Where 
the  leaves  float  upon  the  surface,  as  in  the  various 
water-lilies,  they  are  always  broadly  expanded  and 


270 


EVOLUTION   OF   PLANTS 


usually  almost  circular  in  outline.  Stomata  are  de- 
veloped upon  the  exposed  surface  whose  outer  cell- 
walls  are  also  cutinized,  but  are  absent  from  below. 

Many  submersed  aquatics  have  the  roots  imperfectly 
developed,  serving  merely  as  organs  of  attachment,  or 
they  may  be  quite  absent,  as  in  the  common  bladder- 
weed,  Utricularia.  As  these  plants  absorb  most  of  their 

food  from  the  substances  dis- 
solved in  the  surrounding 
water,  the  roots  are  much  less 
important  than  in  plants  whose 
upper  members  are  exposed  to 
the  air.  However,  the  devel- 
opment in  rootless  forms  of 
special  contrivances  for  pro- 
curing nitrogenous  food,  such 
as  the  traps  of  Utricularia 
(Fig.  58),  would  indicate  that 
the  roots,  even  of  these  sub- 
mersed forms,  are  still  of  im- 
portance in  absorbing  nitrog- 
enous compounds  from  the 
mud  in  which  they  are  fast- 
ened. Where  plants  float 
upon  the  surface,  like  the 
duckweed  (Lemna),  or  Sal- 
vinia,  there  may  be  either 
true  roots  developed,  or  root- 
like  organs  which  replace 
them. 

Most  aquatic  plants  are  entirely  free  from  hairs  or 
scales,  so  that  the  surface  is  smooth.  Exceptions  to 


FIG.  59.  —  A,  two  leaves  of  an 
aquatic  buttercup  (Ranun- 
culus Purshii)  ;  L,  aerial 
leaf;  w,  submersed  leaf; 

B,  twig  of  horse-chestnut, 
showing  a  winter-bud  pro- 
tected by  thick  scales,  sc ; 

C,  plant  of  pine-sap  (Mono- 
tropa),   a   colorless    sapro- 
phyte    with     rudimentary 
leaves,  sc. 


INFLUENCE   OF  ENVIRONMENT  271 

this,  however,  occur  in  a  few  floating  forms,  e.g.  Sal- 
vinia,  Pistia,  which  are  covered  with  hairs,  but  what 
the  significance  of  this  is,  is  hard  to  see. 

XEEOPHYTES 

Very  different  from  the  plants  we  have  been  con- 
sidering are  the  "Xerophytes,"  or  dry-region  plants, 
in  which  are  developed  all  manner  of  devices  for  pre- 
venting loss  of  water,  and  thus  resisting  the  effects  of 
excessive  dryness.  This  has  been  so  successful  that 
very  few  regions  are  so  dry  as  to  be  absolutely  desti- 
tute of  vegetation. 

The  surface  exposed  to  the  air  in  these  xerophytes 
is  much  reduced,  the  leaves  being  either  extremely 
small  or  entirely  absent,  and  the  green  assimilating 
tissue  is  confined  mainly  to  the  stem  and  branches, 
which  may  in  some  cases  become  flattened  and  leaf- 
like,  as  in  many  Acacias.  Where  the  leaves  are  present 
they  are  either  provided  with  very  thick  outer  cells,  so 
that  they  are  hard  and  leathery  in  texture,  like  the 
oleander  or  manzanita,  or  they  are  covered  with  a 
dense  felt  of  hairs,  which  forms  a  most  efficient  pre- 
ventive against  loss  of  water,  and  also  acts  as  a  shield 
against  the  too  powerful  rays  of  the  sun.  Many  desert 
plants  show  this  covering  of  hairs,  which  gives  them 
their  characteristic  gray  color. 

The  xerophytes  are  of  course  most  perfectly  devel- 
oped in  hot  deserts  such  as  those  of  the  southwest- 
ern United  States  and  northern  Mexico,  the  Sahara, 
and  many  parts  of  Australia.  The  traveller  passing 
through  Arizona  and  New  Mexico  will  find  the  vegeta- 


272  EVOLUTION  OF  PLANTS 

tion  thoroughly  xerophytic  in  character.  The  giant 
cacti,  Yuccas,  sage-brush,  and  century  plants  give  the 
scattered  desert  vegetation  a  peculiar  aspect,  which  is 
not  soon  forgotten.  The  cacti  are  probably  as  perfect 
examples  of  adaptation  to  extreme  desert  conditions  as 
can  be  found.  In  these  the  leaves  have  entirely  dis- 
appeared and  the  plant  in  some  forms  is  reduced  to 
a  single,  enormously  enlarged,  often  nearly  globular 
stem,  thus  presenting  the  least  possible  surface,  and 
reducing  the  loss  of  water  to  a  minimum.  The  green 
tissues  are  protected  by  several  overlying  layers  of  cells 
with  thick  walls,  the  outer  ones  strongly  cutinized  so 
as  to  be  waterproof.  Nearly  the  whole  inner  mass  of 
tissue  is  made  up  of  thin-walled  cells  gorged  with 
water,  and  forming  a  reservoir  from  which  the  slight 
loss  of  water  at  the  surface,  due  to  transpiration,  is 
made  good.  Branches  cut  off  and  thrown  upon  the 
ground  will  remain  alive  for  weeks  before  tjie  water 
stored  up  in  them  is  finally  exhausted. 

The  Yuccas  and  the  century  plants  (Agave)  present 
a  type  somewhat  different  from  that  of  the  cacti.  This 
is  best  seen  in  the.  century  plant,  where  the  leaves,  in- 
stead of  being  absent,  are  very  large ;  but  like  the  stem 
of  the  cactus  they  are  enormously  thickened,  and  effi- 
ciently protected  from  loss  of  water  by  the  heavily 
cutinized  walls  of  the  superficial  cells. 

Most  of  these  desert  plants,  as  we  have  indicated  in 
a  former  chapter,  are  very  efficiently  protected  against 
the  attacks  of  herbivorous  animals  by  their  thorny 
armor.  The  terrible  spines  developed  upon  the  cacti, 
and  the  dagger-like  leaves  of  the  Yuccas  and  Agaves, 
are  quite  sufficient  to  keep  the  hungriest  animals  at 


INFLUENCE   OF  ENVIRONMENT  273 

bay.  The  protective  character  of  the  strong  odors 
found  in  many  plants  of  the  same  regions  has  also  been 
referred  to. 

Where  a  region  is  subjected  to  well-marked  wet  and 
dry  seasons,  there  are  always  a  great  many  plants  which 
pass  the  dry  season  in  a  dormant  condition,  very  much 
as  similar  plants  hibernate  during  the  cold  season  of 
more  northern  regions.  These  plants  generally  develop 
bulbs  or  tubers,  which  may  be  completely  dried  up 
without  injury.  Bulbous  plants  are  especially  abun- 
dant in  such  semi-arid  regions  as  central  and  southern 
California,  the  Cape  of  Good  Hope,  and  the  shores  of 
the  Mediterranean,  where  many  bulbous  Monocotyle- 
dons occur,  among  them  some  of  the  choicest  garden 
flowers,  like  the  various  species  of  Narcissus,  Iris, 
Gladiolus,  etc. 

While  these  special  adaptations  to  resisting  dryness 
are  particularly  well  developed  in  the  flowering  plants, 
there  are  also  many  striking  examples  among  the  lower 
plants,  especially  among  Pteridophytes  and  Bryophytes. 
In  California  many  ferns  become  completely  dried  up 
during  the  long  rainless  summer,  but  some  of  them, 
like  the  gold-back  fern  (G-ymnogramme  triangularis), 
on  being  placed  in  water  will  revive  immediately,  the 
dried-up  leaves  unfolding  and  becoming  fresh  and 
green.  The  curious  "resurrection  plant,"  from  the 
southern  part  of  the  state,  is  one  of  the  club-mosses 
(Selaginella  lepidophylla),  and  this  has  the  same  power 
of  rapid  resuscitation.  Many  mosses  and  liverworts 
show  the  same  thing,  the  whole  plant  drying  up 
completely  and  reviving  almost  instantaneously  on 
being  moistened.  Less  commonly  in  these  plants 


274  EVOLUTION  OF   PLANTS 

special  tubers  are  formed,  somewhat  as  in  so  many 
flowering  plants. 

Sometimes  instead  of  having  the  leaves  much  reduced 
in  size,  the  trees  and  shrubs  of  dry,  hot  regions  may 
have  the  position  of  the  leaves  such  as  to  neutralize,  to 
some  extent,  the  power  of  the  sun's  rays.  Instead  of 
being  placed  horizontally,  as  most  leaves  are,  in  this 
class  of  xerophytes  the  leaf  hangs  vertically  and  both 
sides  are  alike.  The  various  species  of  Eucalyptus,  or 
Australian  gum  trees,  show  this  in  a  very  perfect  way, 
and  in  western  America  there  are  a  few  examples,  one 
of  the  best  being  the  manzanita  (Arctostaphylos)  of 
the  California!!  mountains. 

Many  tropical  trees,  whose  leaves  at  maturity  show 
the  normal  position,  have  the  young  leaves  pendent,  so 
that  they  are  protected  from  the  full  force  of  the  sun's 
rays;  these  are  also  very  commonly  colored  pink  or 
crimson  owing  to  the  presence  of  red  cell-sap  in  the 
outer  cells,  and  this  probably  serves  as  a  screen  to 
protect  the  young  chloroplasts. 

It  is  interesting  to  trace  the  development  of  some  of 
these  modifications  as  they  take  place  in  the  growth  of 
the  young  plant.  Thus  the  seedling  Eucalyptus  has 
broad,  horizontal  leaves,  which  also  often  occur  in 
young  shoots  of  the  older  trees,  and  these  are  gradually 
replaced  by  the  pendent  leaves  with  their  vertically  set 
lamina.  In  many  of  the  Australian  Acacias,  where 
the  lamina  of  the  leaf  is  completely  suppressed  in  the 
older  .plant,  and  replaced  by  the  vertically  flattened  leaf- 
stalks, or  phyllodia,  the  young  plant  has  the  feathery 
pinnate  leaves  characteristic  of  so  many  Leguminosse, 
and  the  transition  from  these  to  the  phyllodia  is  very 


INFLUENCE   OF   ENVIRONMENT  275 

gradual.  As  in  Eucalyptus  it  is  not  uncommon  to  find 
a  reversion  to  the  original  leaf -form  on  young  shoots  of 
the  older  trees. 

Similar  in  their  behavior  to  desert  plants  are  the 
"Halophytes,"  those  growing  along  the  seashore  or  in 
salt  marshes.  Thus  the  sea-rocket  (Cakile),  samphire 
(Salicornia),  ice-plant  (Mesembryanthemum),  and  other 
maritime  plants  show  these  peculiarities.  These  plants 
have  fleshy  stems  and  leaves  and  can  live  with  very 
little  water.  The  explanation  of  this  peculiarity  in 
plants  growing  where  there  seems  to  be  an  abun- 
dance of  water,  has  been  thought  to  be  the  fact  that 
the  separation  of  the  water  from  the  salt  solution  is 
difficult,  and,  moreover,  the  accumulation  of  salt  within 
the  tissues  of  the  plants,  if  free  transpiration  of  water 
from  the  surface  took  place,  would  be  injurious  to  the 
plant. 

EPIPHYTES 

Under  the  name  Epiphytes  are  included  those  plants 
which  grow  attached  to  others,  but  are  riot  parasites. 
While  these  epiphytes  usually  grow  upon  trees  or  other 
plants,  not  infrequently  they  may  attach  themselves  to 
rocks  or  other  inanimate  objects.  Epiphytic  plants  are 
most  abundant  in  the  moist,  hot  regions  of  the  tropics, 
but  are  by  no  means  confined  to  these,  since  many 
mosses,  lichens,  and  liverworts  which  occur  plentifully 
in  the  temperate  or  even  arctic  regions,  may  be  prop- 
erly classed  as  epiphytes.  Of  the  ferns  and  fk>wer- 
ing  plants,  however,  very  few  epiphytic  types  occur 
outside  the  tropics,  though  there  they  form  a  most 
characteristic  feature  of  the  vegetation. 


276  EVOLUTION  OF  PLANTS 

These  tropical  epiphytes  represent  many  families  of 
flowering  plants  and  also  include  a  large  number  of 
ferns.  One  family  of  the  latter,  the  exquisite  filmy- 
ferns  (Hymenophyllacese),  are  mainly  epiphytic,  and  one 
of  the  most  charming  sights  of  the  tropical  mountain 
forests  is  exhibited  by  the  trunks  and  branches  of  the 
trees,  covered  with  the  dark-green,  finely  cut  fronds  of 
these  dainty  ferns.  In  these  dark  forests,  reeking  with 
moisture,  everything  is  covered  with  a  mass  of  epi- 
phytic growths,  even  the  leaves  near  the  ground  being 
overgrown  with  lichens  and  creeping  liverworts. 

Of  epiphytic  flowering  plants  there  may  be  recognized 
two  categories  —  the  lianas,  or  creepers,  which,  at  first 
at  least,  are  rooted  in  the  earth,  but  may  later,  by  de- 
veloping aerial  roots,  become  truly  epiphytic;  secondly, 
the  true  epiphytes,  or  "air  plants,"  such  as  many  or- 
chids and  Bromeliacese,  like  the  "  Spanish  moss,"  which 
never  are  connected  with  the  earth.  These  air  plants 
abound  everywhere  in  the  tropical  forests,  and  some  of 
the  epiphytic  orchids  are  among  the  most  beautiful 
of  all  plants.  These  showy  species  are,  however, 
in  a  minority,  as  most  of  the  tropical  orchids  are 
by  no  means  conspicuous.  The  peculiarly  Ameri- 
can family,  the  Bromeliacese,  includes  a  large  num- 
ber of  curious  epiphytes,  some  of  which  are  showy 
plants  with  spiky  leaves  and  large  clusters  of  bril- 
liantly colored  bracts  or  flowers.  The  best  known 
of  these  is  the  "Spanish  moss,"  of  the  southern 
United  States,  but  most  of  them  are  strictly  tropical 
in  their  range. 

Many  species  of  Ficus,  or  fig,  are  epiphytic,  while 
still  others  begin  as  epiphytes,  germinating  upon  the 


INFLUENCE   OF   ENVIRONMENT  277 

branches  of  trees,  and  sending  down  roots  which  finally 
reach  the  ground.  These  roots  as  they  increase  in 
number  and  size  finally  entirely  envelop  the  trunk 
of  the  tree  on  which  the  fig  is  growing,  and  at  last 
actually  strangle  it,  so  that  the  fig  is  left  mounted 
on  a  hollow  trunk  composed  of  the  more  or  less  com- 
pletely joined  roots. 

As  true  epiphytes  have  no  root  system  to  supply 
them  with  water,  and  are  not  connected  with  the  earth, 
various  devices  have  been  developed  for  supplying  them 
with  the  necessary  moisture  and  soil-constituents. 
Many  epiphytic  orchids  develop  bulb-like  enlargements 
of  the  leaf-bases,  which  serve  at  once  for  storing  food 
and  water,  and  may  be  almost  completely  dried  up 
during  their  dormant  season  without  injury.  These 
orchids  frequently  have  long,  fleshy,  aerial  roots, 
which  doubtless  are  important  agents  in  absorbing 
moisture  from  the  air.  Most  of  the  Tillandsias  and 
many  epiphytic  ferns  accumulate  vegetable  mould  in 
their  enlarged  leaf-bases,  which  serve  as  reservoirs  of 
moisture,  and  the  scurfy  scales  with  which  the  leaves 
of  many  species  of  Tillandsia  are  covered  are  also  use- 
ful in  holding  moisture. 

The  various  types  of  climbing  plants  may  be  con- 
sidered in  connection  with  epiphytes.  Like  these  they 
reach  their  greatest  development  in  the  moist  forests  of 
the  tropics,  where  the  struggle  for  existence  is  the 
fiercest.  The  development  of  the  climbing  habit  is 
doubtless  associated  with  the  competition  of  plants  for 
the  light.  In  more  northern  regions,  where  vegetation 
is  less  rank  and  the  crowding  not  so  great,  fewer  plants 
show  this  habit,  but  in  the  dense  tropical  forests  climb- 


278 


EVOLUTION  OF   PLANTS 


ing  plants  are  very  numerous,  and  the  tall  trees  are 
loaded  down  with  giant  creepers  which  are  striving  to 
reach  the  light  overhead.  The  means  by  which  this  is 
accomplished  are  various.  Some  plants  climb  by  twin- 
ing their  slender  stems  about  the  support  (Fig.  60,  A), 
like  the  morning-glory  or  hop ;  others  develop  special 

climbing  organs,  ten- 
drils (B,  C),  which 
are  either  modified 
branches  or  parts  of 
leaves.  The  climb- 
ing rattan  palms,  and 
some  other  tropical 
lianas,  simply  recline 
over  the  branches  of 
trees,  holding  on  by 
stout  hooked  prickles. 
A  smaller  number  of 
FIG.  60  (Climbing  plants). -A,  twining  creepers,  like  the  ivy 

stems  of  Mandevillea  suaveolens  ;  I,  leaf-  g,nd    various    trODlCal 
scar;  B,  leaf  of  sweet-pea  with  the  ter- 
minal divisions  transformed  into  tendrils,  aroids,       C  1 1  HI  b      by 
ten;   C,  twining  leaf-stalk  of  Solarium  ,.      , 

jasminoides.  means  oi   short  root 

tendrils. 


PARASITES  AND  SAPROPHYTES 

Not  to  be  confounded  with  the  epiphytic  plants  are 
the  true  parasites,  such  as  the  mistletoe  and  dodder. 
Some,  like  the  mistletoe  and  its  numerous  tropical  re- 
lations, species  of  Loranthus,  are  only  partially  para- 
sitic, being  provided  with  more  or  less  chlorophyll,  so 
that  they  are  capable  of  carbon  assimilation.  In  the 


INFLUENCE   OF   ENVIRONMENT  279 

case  of  such  complete  parasites  as  the  dodder  (Cuscuta) 
or  the  gigantic  Rafflesia  of  Sumatra,  the  plants  are  quite 
destitute  of  chlorophyll  and  completely  dependent  upon 
the  host  for  their  nourishment.  In  these  the  leaves  are 
reduced  to  scales,  and  the  plant  sends  root-like  suckers 
into  the  host,  or,  in  the  case  of  Rafflesia  and  some  re- 
lated plants,  the  whole  vegetative  part  of  the  parasite 
lives  within  the  host,  like  a  fungus,  and  only  the 
monstrous  flowers  are  borne  upon  the  outside. 

Similar  in  appearance  to  these  parasites  are  a  number 
of  saprophytic  plants  which  get  their  nourishment 
mainly  from  the  decaying  organic  matter  in  vegetable 
mould  or  humus.  Both  leaves  and  roots  in  these  plants 
are  imperfectly  developed  (Fig.  59,  C),  and  in  some 
cases,  at  least,  in  common  with  many  other  plants,  they 
are  intimately  associated  with  a  fungus  in  the  soil 
which  seems  to  supply  them  with  the  food  elements 
derived  from  the  organic  matter  in  the  earth.  The 
curious  Indian  pipe  (Monotropa)  and  its  more  showy 
relation,  the  crimson  snow  plant  (Sarcodes)  of  the 
Sierra  Nevada,  are  examples  of  these  humus  plants. 
In  all  these  parasites  and  saprophytes  there  is  a  marked 
degeneration  of  the  assimilating  organs,  and  this  often 
extends  to  other  parts  of  the  plant,  including  the  ovules 
and  embryo. 

SYMBIOSIS 

A  curious  association  of  two  plants  together,  or  less 
often  of  a  plant  and  animal,  is  a  not  uncommon  occur- 
rence, this  "symbiosis"  being  apparently  mutually 
beneficial,  although  sometimes  it  looks  more  like  a  case 
of  parasitism.  A  number  of  liverworts,  e.g.  Blasia, 


280  EVOLUTION  OF  PLANTS 

Anthoceros,  and  others,  always  have  within  the  thallus 
colonies  of  a  low  blue-green  alga,  Nostoc,  and  the 
little  water  fern,  Azolla,  has  in  each  leaf  a  cavity  con- 
taining a  colony  of  a  similar  alga,  Anabsena.  Just 
what  are  the  mutual  relations  of  the  plants  in  these 
cases  has  not  been  clearly  made  out. 

Somewhat  different  is  the  case  of  the  lichens,  where 
various  low  algse,  such  as  Protococcus  or  Nostoc,  are 
included  in  a  thallus  whose  principal  constituent  is  a 
sac-fungus,  whose  hyphae  are  closely  united  with  the 
green  cells  of  the  algae,  and  which  is  incapable  of  de- 
velopment if  the  algal  cells  are  absent.  The  latter, 
however,  grow  perfectly  well  when  removed  from  the 
lichen  thallus,  and  it  is  doubtful  whether  they  benefit, 
to  any  great  extent,  from  their  association  with  the 
fungus,  except  as  they  are  sheltered  and  perhaps  pro- 
tected from  excessive  drying.  Somewhat  similar  is  the 
association  of  minute  unicellular  algae  with  some  of  the 
lower  animals,  e.g.  Paramoecium,  Spongilla,  Hydra,  etc. 

Among  the  most  important  cases  of  symbiosis  are 
those  existing  between  various  organisms  in  the  soil 
and  the  roots  of  flowering  plants.  The  most  noteworthy 
of  these  organisms  are  the  nitrifying  bacteria  which  are 
the  principal  agents  in  the  preparation  of  nitrogenous 
matter  in  the  soil,  so  that  it  is  available  for  the  higher 
plants.  These  peculiar  organisms  sometimes  associate 
themselves  directly  with  the  plants,  this  being  espe- 
cially noticeable  in  the  Leguminosae,  which  are  notably 
rich  in  nitrogen.  In  these,  e.g.  pea,  clover,  lupine, 
etc.,  there  are  developed  upon  the  roots  little  tubercles 
within  which  are  great  numbers  of  minute  bacteria,  to 
whose  activity  is  due  the  assimilation  of  nitrogenous 


INFLUENCE   OF  ENVIRONMENT  281 

matter,  to  a  certain  extent  the  free  nitrogen  of  the 
atmosphere,  which  otherwise  is  quite  unavailable  for 
plant  food. 

Finally,  there  is  always  found  in  connection  with  the 
roots  of  many  trees,  especially  the  Cupuliferse  (oaks, 
beeches,  etc.),  certain  fungus  filaments,  or  "mycor- 
rhiza,"  which  appear  to  take  the  place  of  root-hairs,  and 
while  parasitic  to  some  extent  upon  the  roots,  neverthe- 
less are  of  great  importance  to  their  host  in  supplying 
it  with  food  from  the  soil. 

PROTECTION  AGAINST  COLD 

So  far  as  can  be  judged  from  the  geological  evidence, 
the  temperature  of  the  earth  was  formerly  more  uniform 
than  at  present,  and  consequently  the  flora  was  also 
more  uniform  and  composed  of  typej  which  now  belong 
to  the  temperate  or  sub-tropical  zones.  It  is  likely  that 
a  large  part  of  these  plants  were  evergreen,  as  is  now 
the  case  in  the  warmer  parts  of  the  world.  As  the  cli- 
mate grew  more  severe  with  the  oncoming  of  the  glacial 
epoch,  it  is  probable  that  the  deciduous  habit  was  de- 
veloped in  response  to  this,  the  only  evergreen  trees  of 
high  latitudes  at  present  being  the  Conifers,  most  of 
which  have  retained  their  primitive  evergreen  habit. 

Where  there  is  each  year  a  long  period  of  cold 
weather,  during  which  growth  ceases  entirely,  it  is 
clear  that  trees  with  broad  leaves,  exposed  to  the  severe 
cold,  and  to  loss  of  water  by  evaporation,  are  at  a  great 
disadvantage  compared  to  those  which  shed  their  leaves 
at  the  end  of  the  growing  period  and  whose  dormant 
buds  are  thoroughly  protected  by  the  thick  scales  de- 
veloped about  the  winter  buds  of  all  deciduous  woody 


282  EVOLUTION  OF  PLANTS 

plants  (Fig.  59,  B).  These  trees  and  shrubs,  with  all 
their  delicate  tissues  carefully  protected  against  the 
effects  of  severe  cold,  can  endure  without  harm  a  tem- 
perature which  would  quickly  destroy  any  plant  with 
broad  evergreen  leaves. 

Most  perennial  herbaceous  plants  of  cold  climates 
also  have  special  provision  against  the  cold  in  the  de- 
velopment of  underground  parts,  bulbs,  tubers  or  root- 
stocks,  which  remain  dormant  during  the  winter  and 
send  up  their  shoots,  which  grow  very  rapidly  at  the 
expense  of  the  reserve  food  stored  in  these  subter- 
ranean reservoirs,  so  soon  as  the  first  warm  weather  of 
spring  starts  them  into  growth. 

MOVEMENTS  OF  PLANTS 

We  have  seen  that  the  lowest  plants  are  actively 
motile  and  how  this  motility  has  been  retained  by  the 
reproductive  cells  in  all  but  the  highest  ones.  The 
power  of  spontaneous  movement  is  common,  however, 
to  the  protoplasm  of  all  plants,  and  in  the  higher  plants 
movements  of  various  organs  are  sufficiently  familiar 
phenomena.  These  movements  are,  to  a  considerable 
extent,  responses  to  external  stimuli.  The  bending  of 
growing  parts  of  plants  to  the  light,  and  the  effect  of 
light  and  temperature  upon  the  opening  and  closing 
of  many  flowers  are  everyday  occurrences.  Not  so 
familiar,  except  to  the  botanist,  are  the  revolving  move- 
ments of  growing  tips,  especially  in  twining  plants, 
which  are  among  the  most  important  factors  in  the 
twining.  Many  tendrils  are  exceedingly  sensitive  to 
contact,  curving  quickly  in  response  to  this  pressure, 


INFLUENCE   OF  ENVIRONMENT  283 

and  no  doubt  this  extreme  sensitiveness  is  an  advantage 
to  the  plant. 

We  have  already  spoken  of  the  development  of  sen- 
sitiveness resulting  in  the  movements  of  various  parts  of 
the  flower,  in  connection  with  the  subject  of  pollination, 

The  movements  of  leaves  in  response  to  stimuli  of 
various  kinds  are  especially  developed  in  several  groups 
of  plants,  of  which  the  Leguminosse  are  perhaps  the 
most  notable.  The  well-known  sensitive  plant  (Mi- 
mosa) is  the  best  known  of  these,  but  many  common 
leguminous  plants,  like  the  species  of  clover,  locust, 
beans,  and  many  others,  exhibit  marked  movements  of 
the  leaves,  being  especially  sensitive  to  changes  in  the 
intensity  of  the  light  to  which  they  are  exposed.  Thus 
most  of  these  plants  have  the  leaves  folded  up  at  night, 
exhibiting  the  so-called  "sleep  movements." 

Movements  of  a  purely  mechanical  kind  occur  in 
many  plants,  both  among  the  lower  ones  and  the 
flowering  plants.  The  hygroscopic  movements  of  the 
elaters  of  liverworts,  or  the  peristome-teeth  of  the  moss 
capsule,  the  opening  of  the  sporangia  of  the  ferns  and 
of  the  anthers  of  flowers,  are  all  good  examples  of  this. 
These  movements  are  entirely  due  to  the  unequal  ab- 
sorption of  water  by  the  cell-walls  of  the  motile  organ, 
or  to  unequal  loss  of  water  from  them.  Similar  hygro- 
scopic movements  are  exhibited  by  the  awns  of  grasses 
and  those  attached  to  the  fruits  of  other  plants,  e.g.  the 
spirally  twisted  awn  of  the  fruit  in  alfilaria  (Erodium). 
The  opening  of  most  seed-vessels,  such  as  those  of  the 
violet  or  balsam,  are  of  much  the  same  nature.  All  of 
these  movements  are  connected  with  the  dispersal  of 
the  spores  or  seeds. 


CHAPTER    XV 

SUMMARY  AND   CONCLUSION 

ALL  plants  agree  closely  in  their  essential  cell  struct- 
ure, the  typical  cell  having  a  cellulose  membrane  and 
a  single  nucleus.  This  simple  type  of  cell  constitutes 
the  whole  plant  in  many  low  forms  and  makes  up  the 
young  parts  of  the  higher  plants.  From  it  are  derived 
the  variously  modified  cell  forms  constituting  the  spe- 
cialized tissues  of  these  higher  plants.  In  the  lowest  of 
all  plants,  the  Bacteria  and  their  blue-green  allies  the 
Sehizophycese,  the  cell  does  not  always  show  a'cellulose 
membrane  and  the  nucleus  is  imperfectly  developed. 

The  lowest  plants  are  mainly  aquatic,  and  it  is  ex- 
ceedingly probable  that  this  is  the  primitive  condition 
for  plant  life.  Leaving  aside  the  Schizophytes,  whose 
affinities  are  somewhat  doubtful,  the  peculiar  group  of 
motile  green  algse,  the  Volvocineae,  probably  represents 
more  nearly  than  any  existing  forms  the  ancestral  type 
of  all  the  higher  green  plants.  These  ciliated  algae 
are  also  probably  related  to  certain  colorless  flagellate 
Infusoria,  which  in  turn  may  represent  the  starting- 
point  for  the  whole  group  of  Metazoa  among  the  ani- 
mals. It  is  not  unlikely  that  the  separation  of  the  two 
great  branches  of  organisms,  plants  and  animals,  took 
place  among  the  Flagellata. 

284 


SUMMARY   AND  CONCLUSION  285 

One  important  reason  for  considering  the  ciliated 
Volvocinese  as  primitive  forms  in  the  very  frequent 
reversion  to  this  condition  exhibited  at  times  by  very 
many  of  the  higher  green  plants,  whose  reproductive 
cells,  zoospores,  and  gametes  very  generally  are  extraor- 
dinarily similar  in  structure  to  the  simpler  Volvocinese. 
The  persistence  of  motility  in  the  reproductive  cells 
is  very  remarkable,  being  found  in  members  of  all  the 
groups.  The  spermatozoids  of  the  Archegoniates  — 
mosses  and  ferns  —  illustrate  this,  and  the  recent  dis- 
covery of  similar  motile  cells  in  the  lowest  of  the  seed 
plants  extends  this  phenomenon  to  the  highest  sub- 
kingdom. 

Starting  with  this  primitive  motile  unicellular  organ- 
ism, there  have  evidently  arisen  a  number  of  indepen- 
dent lines  of  development  resulting  in  very  divergent 
types  of  structure.  The  first  step  in  the  evolution  of 
what  may  be  termed  the  typical  green  plants  is  the  loss 
of  motility  in  the  vegetative  cells  through  the  suppres- 
sion of  the  cilia  and  the  development  of  a  firm  cell-wall. 
The  latter  precludes  the  active  locomotion,  so  charac- 
teristic of  most  animal  forms,  and  makes  the  plant 
assume  the  more  stable  condition  typical  of  the  vege- 
table organism. 

In  these  lowly  organisms  there  is  no  clearly  marked 
line  between  vegetative  and  reproductive  cells.  An 
individual  by  simple  fission  gives  rise  to  two  new  in- 
dividuals like  itself.  Many  of  these,  however,  show 
two  kinds  of  cell-division,  a  purely  vegetative  one  by 
fission  into  two  equal  parts,  and  a  modification  of  this, 
internal  cell-division,  by  which  a  number  of  individuals 
may  arise  by  simultaneous  division  of  the  protoplasm  of 


286  EVOLUTION   OF   PLANTS 

the  mother-cell  after  a  preliminary  division  of  the 
nucleus  into  as  many  secondary  nuclei.  In  the  latter 
case,  the  resulting  cells  may  be  non-sexual,  or  they 
may  exhibit  the  simplest  form  of  sexual  reproduction, 
i.e.  the  cells  may  be  similar  gametes  which  unite  in 
pairs  preliminary  to  the  formation  of  new  individuals. 
These  reproductive  cells  are  usually  motile  and  closely 
resemble  the  ancestral  Volvox  cell. 

If  the  two  cells  resulting  from  the  fission  of  a  uni- 
cellular organism  remain  together,  and  this  is  re- 
peated, there  results  a  cell-complex,  the  simplest  type 
being  the  cell-row  found  in  so  many  of  the  green 
algse,  like  Spirogyra  or  Conferva.  The  next  step  in 
advance  is  the  formation  of  filaments,  like  those  of 
CEdogonium,  with  definite  base  and  apex,  the  filament 
usually  being  attached  by  a  simple  holdfast.  Next  by 
division  in  two  planes  is  formed  such  a  simple  flat 
thallus  as  that  of  Coleochaate.  So  far  as  is  known  at 
present,  this  is  the  highest  type  the  plant  body  assumes 
among  the  Chlorophycese  or  green  algae  except  in  the 
case  of  the  Characese,  whose  affinities  with  the  other 
algse  are  doubtful.  From  some  forms  probably  not 
unlike  Coleochsete,  the  lowest  of  the  mosses  were 
derived. 

The  increasing  complexity  of  the  plant  body  has 
been  accompanied  by  a  corresponding  specialization  of 
the  reproductive  parts.  Most  of  the  green  algae  have 
both  sexual  and  non-sexual  reproductive  cells,  the  latter 
most  commonly  being  motile  zoospores.  The  lower 
members  of  the  series  have  the  gametes,  or  sexual  cells, 
alike,  but  in  the  higher  ones  the  female  gamete,  or 
egg,  loses  the  power  of  movement  and  is  retained  within 


SUMMARY   AND   CONCLUSION  287 

a  special  cell,  or  oogonium,  where  it  is  fertilized  by  the 
much  smaller  motile  spermatozoid. 

Besides  this  line  of  Chlorophyceee,  which  may  be  as- 
sumed to  have  given  rise  to  the  Bryophytes,  there  are 
several  other  groups  which  have  branched  off  from  the 
primitive  stock.  The  most  important  of  these  are  the 
Siphonese,  characterized  by  the  complete  suppression 
of  division  walls  in  the  often  large  thallus;  and  the 
two  very  important  groups  of  marine  algse,  the  red  and 
the  brown  sea-weeds,  characterized  by  the  special  pig- 
ments developed,  as  well  as  other  important  peculiari- 
ties. It  is  among  these  marine  algse  that  there  are 
found  the  largest  and  most  conrplex  of  the  Thallophytes, 
but  this  is  not  always  associated  with  a  corresponding 
perfection  of  the  reproductive  parts,  which  may  be 
exceedingly  primitive.  Thus  in  the  giant  kelps,  often 
hundreds  of  feet  in  length,  so  far  as  is  known  only 
non-sexual  zoospores  of  the  simplest  description  are 
developed. 

These  great  sea-weeds  have  been  profoundly  modified 
by  their  environment  and  have  diverged  widely  in  their 
structure  from  the  primitive  fresh-water  forms  which,  in 
another  direction,  have  given  rise  to  the  higher  plants. 
It  is  exceedingly  unlikely  that  either  the  red  or  the 
brown  algae  have  produced  any  higher  types,  but  they 
themselves  represent  the  highest  expression  of  their 
respective  lines  of  development. 

The  evolution  of  the  sexual  cells,  i.e.  the  transition 
from  the  non-sexual  zoospores,  first  to  similar  gametes, 
and  later  to  the  separate  male  and  female  cells,  has  evi- 
dently been  accomplished  quite  independently  in  several 
'widely  separated  groups  of  plants, —  e.g.  Volvocinese, 


288  EVOLUTION  OF  PLANTS 

Confervacese,  Siphonese,  and  Pheeophycese, —  so  that  the 
possession  of  sexual  cells  showing  a  similar  grade  of 
development  does  not  by  any  means  necessarily  imply 
relationship. 

The  origin  of  the  Phseophycese,  or  brown  algse,  from 
free-swimming  brown  flagellate  organisms,  is  by  no 
means  unlikely,  and  if  this  is  shown  to  be  the  case,  they 
must  be  considered  as  a  line  of  development  parallel 
with  the  Chlorophycese  rather  than  an  offshoot  from 
these.  It  may  also  be  said  of  the  red  algae,  that  they 
may  possibly  constitute  an  entirely  independent  devel- 
opmental line,  but  this  is  less  likely  than  in  the  case 
of  the  Phseophycese. 

The  relationships  of  the  Fungi  is  still  an  open  ques- 
tion. Certain  forms,  the  Phycomycetes  or  alga-fungi, 
especially  the  water-moulds  and  their  allies,  so  closely 
resemble  such  siphoneous  alg^e  as  Vaucheria,  both  in 
the  structure  of  the  thallus  and  in  the  character  of  the 
reproductive  cells,  as  to  leave  little  doubt  of  their 
probable  derivation  from  some  such  green  ancestral 
forms.  These  Phycomycetes  may  be  said  to  bear 
much  the  same  relation  to  these  green  algse  that  such 
parasites  and  saprophytes  as  the  dodder  and  Indian 
pipe  do  to  their  green  relatives  among  flowering  plants. 

The  question  of  the  relation  of  the  true  Fungi,  or 
Mycomycetes,  to  these  alga-fungi,  is  by  no  means  so 
clear,  although  it  is  generally  supposed  that  they  have 
been  derived  from  some  such  forms.  Some  authorities 
claim,  however,  that  the  two  groups  are  quite  inde- 
pendent of  each  other,  and  that  the  line  of  Mycomycetes 
has  originated  from  chlorophyll-less  plants  of  extremely 
simple  structure. 


SUMMARY  AND   CONCLUSION  289 

The  ancestors  of  the  higher  green  plants  must  be 
sought  among  the  simple  fresh-water  green  algae.  The 
genus  Coleochsete,  the  most  specialized  of  the  Confer- 
vaceae,  is  the  form  which  shows  the  nearest  analogy 
with  the  lower  Bryophytes,  which  it  closely  resembles 
in  the  development  of  a  rudimentary  sporophyte  as  the 
result  of  fertilization,  and  thus  shows  a  very  simple 
case  of  the  alternation  of  generations  so  characteristic 
of  all  Archegoniates.  In  the  mosses  this  becomes  well 
marked,  but  there  is  a  good  deal  of  difference  between 
the  simplest  of  these  and  the  highest  green  algae, 
although  the  persistence  of  the  motile  spermatozoids 
indicates  the  derivation  of  the  Archegoniates  from 
aquatic  ancestors. 

The  mosses,  being  mainly  terrestrial  plants,  have 
developed  much  more  perfect  tissues  than  the  Algae,  and 
in  the  ferns,  which  undoubtedly  are  related  to  them, 
this  is  still  more  marked.  In  both  groups  of  Arche- 
goniates, the  reproductive  organs,  archegonia  and  an- 
theridia,  agree  closely  in  structure,  and  the  sporophyte 
always  gives  rise  to  spores  which  are  formed  in  tetrads 
from  a  common  mother-cell. 

The  Mosses  (Bryophytes)  show  two  well-marked 
series,  or  classes,  Hepaticae,  or  liverworts,  and  Musci, 
or  true  mosses.  The  former  are  the  more  primitive  and 
show  many  points  of  resemblance  to  the  Chlorophyceae, 
and  they  are  especially  important  as  being  the  primitive 
stock  from  which  the  several  series  of  archegoniate 
plants  have  diverged,  bearing  much  the  same  relation 
to  these  higher  Archegoniates  that  the  green  algae  do 
to  the  Thallophytes. 

In  the  lower  liverworts,  the  sporophyte,  which  arises 


290  EVOLUTION   OF   PLANTS 

from  the  fertilized  egg-cell,  is  very  simple  in  structure, 
and  is  devoted  almost  exclusively  to  spore-production, 
having  no  power  of  independent  growth,  but  living  as 
a  parasite  upon  the  tissues  of  the  gametophyte.  Within 
the  Hepaticse,  however,  are  forms  in  which  the  sporo- 
phyte  becomes  much  more  important,  and  in  the  genus 
Anthoceros,  especially,  it  reaches  a  large  size  and 
becomes  almost  independent  of  the  gametophyte  owing 
to  the  development  of  several  layers  of  green  tissue 
communicating  with  the  atmosphere  by  means  of  sto- 
mata,  exactly  as  in  the  higher  plants.  Here,  too,  only 
a  small  part  of  the  tissue  is  devoted  to  spore-formation, 
and  the  growth  of  the  sporophyte  does  not  cease  as  soon 
as  the  first  spores  are  ripe.  No  root,  however,  is  de- 
veloped, and  the  sporophyte  remains  dependent  upon 
the  gametophyte  for  its  supply  of  water  and  for  such 
food  elements  as  it  cannot  obtain  from  the  air.  The 
duration  of  its  growth  is  therefore  determined  by 
that  of  the  gametophyte. 

The  gametophyte  in  the  Bryophytes  may  reach  a  very 
considerable  size,  and  is  sometimes  quite  complicated 
in  its  structure,  but  this  does  not  necessarily  corre- 
spond to  the  development  of  the  sporophyte,  which 
reaches  its  highest  expression  in  forms  with  a  very 
simple  gametophyte. 

It  is  in  the  Pteridophytes,  or  ferns,  that  the  sporo- 
phyte first  becomes  entirely  self-supporting.  Here  the 
embryo-sporophyte  closely  resembles  that  of  the  mosses, 
but  soon  develops  the  special  organs,  stem,  root,  and 
leaf,  which  distinguish  the  fern-sporophyte  and  render 
it  independent  of  the  gametophyte,  which  now  withers 
away  as  soon  as  the  young  sporophyte  is  established. 


SUMMARY  AND   CONCLUSION  291 

The  sporophyte  here  is  a  much  more  highly  organized 
structure  than  the  gametophyte,  reversing  the  relation 
of  these  as  found  in  the  mosses.  In  the  ferns  it  is  the 
sporophyte  which  is  had  in  mind  when  a  fern  is  spoken 
of.  The  gametophyte  (prothallium)  is  inconspicuous 
and  usually  of  brief  duration,  but  it  must  be  borne  in 
mind  that  the  leafy  fern  plant,  even  the  gigantic  tree 
fern,  is  morphologically  the  equivalent  of  the  moss 
capsule,  or  the  still  simpler  sporogonium  of  the  lower 
liverworts. 

It  is  quite  possible  that  the  development  of  an  inde-4 
pendent  sporophyte  has  taken  place  at  more  than  one 
point,  and  that  the  different  series  of  Pteridophytes 
have  not  all  originated  from  a  common  stock.  The 
biciliate  spermatozoids  of  the  club-mosses  and  the  mul- 
ticiliate  ones  of  the  other  Pteridophytes  favor  this  view, 
although  all  of  the  existing  Pteridophytes  closely  re- 
semble each  other  in  the  character  of  their  reproductive 
parts. 

Corresponding  to  the  external  differentiation  of  the 
sporophyte,  there  is  a  much  greater  diversity  in  the  tis- 
sues of  the  Pteridophytes  than  is  found  in  any  of  the 
lower  plants,  this  being  especially  shown  in  the  devel- 
opment of  the  complicated  vascular  bundles.  The 
spores,  too,  are  here  restricted  to  a  special  organ,  the 
sporangium. 

The  Pteridophytes,  also,  show  traces  of  an  aquatic 
ancestry  in  the  development  of  spermatozoids,  which 
require  water  in  order  that  they  may  reach  the  arche- 
gonium,  so  that  it  is  necessary  for  the  gametophyte  to 
be  covered  with  water  in  order  to  insure  fertilization. 

With  the  increasing  importance  of  the  sporophyte, 


292  EVOLUTION  OF  PLANTS 

there  is  a  gradual  reduction  of  the  gametophyte.  This 
in  the  lower  forms  is  long  lived  and  much  like  the 
simpler  liverworts  in  its  structure,  and  bears  both  arche- 
gonia  and  antheridia.  Other  forms  develop  male  and 
female  gametophytes  from  similar  spores,  and,  finally, 
heterospory  has  arisen  in  several  groups  of  Pterido- 
phytes.  In  these,  two  sorts  of  spores  are  produced 
which  on  germination  give  rise  respectively  to  exceed- 
ingly reduced  male  or  female  plants.  Heterospory  is 
found  in  several  groups  of  living  ferns,  and  in  one 
genus,  Selaginella,  among  the  club-mosses.  It  is  evi- 
dent from  a  study  of  fossil  Pteridophytes  that  it  was 
also  developed  in  the  Equisetinese.  In  Selaginella  the 
germination  of  the  spores  begins  within  the  sporangium, 
which  sometimes  falls  away  with  the  contained  spores. 

The  permanent  retention  of  the  spores  within  the 
sporangium  until  the  germination  is  complete,  and  the 
thickening  of  the  sporangium-wall  as  a  protection  to 
the  included  gametophyte  and  embryo,  the  whole  finally 
becoming  detached  from  the  sporophyte,  is  the  origin 
of  the  seed  of  the  higher  plants,  which  is  therefore  only 
a  further  development  of  the  macrosporangium  of  the 
heterosporous  Pteridophytes. 

In  the  seed  plants,  or  flowering  plants,  the  reduction 
of  the  gametophyte  reaches  its  extreme,  but  there  is  no 
absolute  break  between  Pteridophytes  and  Spermato- 
phytes.  The  retention  of  the  germinating  macrospore 
within  the  sporangium  has  necessitated  a  different 
method  of  fertilization,  hence  the  development  of  the 
pollen-tube.  The  lower  Spermatophytes,  especially  the 
Cycads,  while  developing  a  pollen-tube  from  the  ger- 
minating microspore,  nevertheless  produce  spermato- 


SUMMARY  AND  CONCLUSION  293 

zoids  within  this,  which  are  discharged,  with  the 
contained  water,  into  the  cavity  above  the  archegonium, 
and  fertilize  the  latter  in  the  same  way  as  among  the 
Pteridophytes. 

Comparing  the  homologies  of  the  higher  Pteridophytes 
and  the  flowering  plants,  we  find  that  both  produce  two 
sorts  of  sporangia,  macrosporangia  and  microsporangia, 
known  usually  among  the  latter  group  as  ovules  and 
pollen-sacs.  In  the  latter,  spores  develop  precisely  as 
in  all  the  Archegoniates  from  the  lowest  to  the  highest, 
i.e.  by  the  division  of  each  sporogenous  cell  into  four 
spores.  The  macrosporangium,  or  ovule,  of  the  Sper- 
matophytes  generally  contains  but  a  single  macrospore, 
or  embryo-sac,  although  there  are  some  exceptions  to 
this  rule.  Very  often  one  or  both  of  the  preliminary 
divisions  in  the  sporogenous  cell  are  suppressed.  The 
sporangia  of  the  Spermatophytes  are  usually  borne  upon 
sporophylls  —  carpels  or  stamens  —  which  are  the  homo- 
logues  of  the  sporophylls  of  the  Pteridophytes. 

Of  the  Spermatophytes,  the  Gymnosperms  are  obvi- 
ously the  lowest  types,  i.e.  they  show  more  clearly  their 
derivation  from  the  Pteridophytes.  Their  more  primi- 
tive character  is  borne  out  both  by  a  study  of  their  struct- 
ure and  by  their  geological  history.  It  is  not  likely 
that  all  the  Gymnosperms  constitute  a  homogeneous 
class.  It  i&  much  more  probable  that  they  represent 
the  remnants  of  two,  and  possibly  more,  quite  distinct 
developmental  lines.  The  Cycads  show  close  affinity 
with  the  true  ferns,  while  the  Conifers  recall  more 
strongly  the  t^ycopods.  Both  of  these  groups,  espe- 
cially the  Cycads,  are  much  less  abundant  at  the  present 
time  than  in  earlier  periods  of  the  earth's  history. 


29-i  EVOLUTION   OF  PLANTS 

The  Angiosperms  are  preeminently  the  modern  plant 
type.  These  have  largely  crowded  out  the  other  earlier 
types  of  vegetation,  and  at  present  comprise  a  large 
majority  of  existing  species.  In  the  earlier  geological 
formations,  Pteridophytes  and  Gymnosperms  predomi- 
nated; but  as  the  later  formations  are  examined,  the 
Angiosperms  become  more  and  more  important,  prob- 
ably first  appearing  in  the  Mesozoic  age  and  rapidly 
increasing  in  number  and  variety  in  the  more  recent 
formations. 

It  is  among  the  Angiosperms  that  the  plant  body 
reaches  its  highest  expression.  In  the  keen  struggle 
for  existence  among  the  manifold  forms  of  plants,  the 
Angiosperms  have  shown  themselves  to  be  extraordina- 
rily plastic,  and  have  developed  every  possible  device 
to  enable  them  to  survive  this  fierce  competition.  This 
is  especially  shown  in  the  extraordinary  variety  of  the 
floral  structures  to  which  they  have  given  rise.  The 
primitive  flowers  were  doubtless  very  inconspicuous 
and,  as  in  the  case  of  many  existing  flowers  of  similar 
character,  were  dependent  upon  the  wind  or  upon  cur- 
rents of  water  for  conveying  the  pollen  to  the  stigma. 
This  uncertain  mode  of  pollination  involves  a  great 
waste  of  pollen,  and  evidently  any  device  which  insures 
a  saving  of  pollen  is  advantageous.  This  has  been  ac- 
complished by  the  adoption  of  insect  aid  in  pollination. 
This  probably  began  by  the  casual  visits  of  insects  to 
flowers  for  their  pollen,  some  of  which  was  transferred 
to  the  pistil  of  the  next  flower  visited.  Any  flower 
which,  by  reason  of  its  brighter  color  or  stronger  odor, 
made  itself  more  noticeable  to  insects  searching  for  pol- 
len, would  naturally  stand  a  better  chance  of  being  vis- 


SUMMARY  AND   CONCLUSION  295 

ited  by  insects,  and  thus  of  insuring  cross-fertilization, 
which  appears  to  be  distinctly  advantageous  to  the 
plant.  From  these  probably  accidental  variations  have 
been  developed  the  mechanical  devices  for  insuring 
cross-fertilization,  as  well  as  infinite  varieties  of  color 
and  form,  and  the  production  of  nectar  and  odors,  serv- 
ing as  lures  to  attract  insects.  The  extraordinary 
development  of  the  Angiosperms  and  Insects,  the  two 
largest  divisions  of  the  vegetable  and  animal  kingdoms 
respectively,  is  to  a  very  great  degree  correlated,  the 
two  groups  being  largely  dependent  upon  each  other 
for  their  existence. 

While  provision  for  the  development  of  seed  is  one 
of  the  most  important  functions  of  the  plant,  their  dis- 
tribution is  also  necessary,  and  many  arrangements  for 
this  have  been  evolved.  The  development  of  edible 
seeds  and  fruits,  and  of  the  numerous  organs  like  the 
wings  of  such  fruits  as  those  of  the  maple  and  ash,  or 
the  down  in  the  thistle  or  milkweed,  the  hooks  and 
prickles  upon  the  fruits  of  many  Compositse  and  Bor- 
raginese,  are  all  devices  for  facilitating  the  distribution 
of  the  seeds  through  the  agency  of  the  wind  or  by 
animals. 

Extensive  modifications  have  arisen  in  the  plant  by 
which  it  adapts  itself  to  a  changed  environment  or  pro- 
tects itself  against  the  attacks  of  animal  enemies.  The 
earliest  plants  were  probably  aquatic,  and  their  de- 
scendants, but  little  changed,  still  exist  in  the  low 
green  algse.  The  change  from  fresh  to  salt  water  has 
110  doubt  changed  the  marine  forms  profoundly,  this 
being  especially  marked  in  the  red  and  the  brown  algae, 
which  differ  widely  from  their  probably  more  primitive 


296  EVOLUTION  OF  PLANTS 

green  relatives  of  fresh  water;  but  the  modifications 
found  in  the  Algse  are  slight  when  "compared  with  the 
profound  structural  changes  exhibited  by  the  Arche- 
goniates  and  Spermatophytes,  which  have  become 
adapted  to  terrestrial  life. 

With  the  change  from  the  aquatic  to  the  terrestrial 
environment  the  tissues  have  become  very  much  better 
developed,  especially  the  mechanical  tissues  which  give 
rigidity  and  strength  to  the  plant.  The  difference  in 
the  degree  to  which  these  are  developed  in  closely 
related  land  and  water  plants  is  very  noticeable,  and 
is  of  course  directly  associated  with  the  changed 
environment. 

The  degree  of  moisture  varies  extremely  over  land 
areas,  and  those  plants  which  inhabit  dry  regions  have 
become  much  changed,  so  that  they  are  enabled  to 
endure  extreme  dr}oiess,  either  by  having  the  surface 
exposed  to  the  dry  atmosphere  much  reduced  through 
the  partial  or  complete  suppression  of  the  leaves,  or  by 
having  these  very  perfectly  protected  against  loss  of 
water  by  means  of  extremely  thick  impervious  cells  upon 
the  outside,  or  by  a  thick  covering  of  hairs  or  scales. 
Other  xerophytes,  or  dry-region  plants,  are  character- 
ized by  thickened  underground  stems  which  serve  as 
reservoirs  of  water,  or  remain  dormant  during  the  dry 
period,  starting  quickly  into  growth  with  the  advent 
of  the  brief  rainy  season. 

Plants  which  are  subject  to  extreme  cold  have  devel- 
oped protective  structures  similar  to  those  of  plants 
whose  growth  is  checked  by  drought.  These  plants, 
too,  often  develop  underground  resting  stems,  which 
send  up  the  annual  shoots  when  spring  arrives.  The 


SUMMARY  AND  CONCLUSION  297 

deciduous  leaves  and  winter  buds  of  the  woody  plants 
of  cold  regions  are,  with  little  question,  adaptations  of 
a  similar  nature. 

Normal  green  plants  alone  are  capable  of  utilizing  the 
carbon  dioxide  of  the  atmosphere,  and  those  plants 
which  have  no  chlorophyll  must  depend  upon  either 
living  or  dead  organic  matter  for  their  carbonaceous 
food.  Among  the  flowering  plants,  at  least,  these 
parasites,  or  saprophytes,  are  always  evidently  related 
to  normal  green  forms,  and  are  unquestionably  second- 
ary forms  which  are  descended  from  chlorophyll-bearing 
plants.  These  parasites  always  show  evidences  of  more 
or  less  profound  degeneration,  the  leaves  and  roots  usu- 
ally being  rudimentary,  and  the  floral  parts  often  shar- 
ing in  this  degeneration.  This  degradation  of  the 
reproductive  parts  in  parasitic  and  saprophytic  plants 
is  especially  noticeable  in  fungi,  where  in  many  in- 
stances all  traces  of  the  sexual  reproductive  parts  are 
apparently  lost.  Among  the  flowering  plants,  the  seeds 
of  such  forms  are  often  very  small  and  the  embryo 
rudimentary. 

Since  light  is  of  the  first  importance  to  all  plants 
possessing  chlorophyll,  many  adaptations  are  associated 
with  this.  Epiphytes  and  climbing  plants  of  various 
kinds  have  developed  their  special  habits  of  growth  in 
response  to  the  need  of  light.  So  also  the  development 
of  special  pigments  associated  with  the  chlorophyll  is, 
in  most  cases,  to  be  explained  as  being  concerned  with 
the  question  of  light. 

In  short,  we  find  that  plants  have  succeeded  in  adapt- 
ing themselves  to  almost  every  environment.  From 
the  open  ocean  to  arid  deserts  and  lofty  mountain  tops 


298  EVOLUTION   OF  PLANTS 

some  plants  have  succeeded  in  establishing  themselves, 
and  from  the  equator  to  the  poles  no  district  is  com- 
pletely wanting  in  some  types  of  vegetable  life. 

Starting  from  indifferent  unicellular  organisms,  in- 
termediate in  character  between  plants  and  animals, 
we  have  seen  how  there  has  been  a  steady  progression 
in  the  direction  of  the  more  specialized  plants.  This 
progression  consists  in  specialization  of  both  vegetative 
and  reproductive  parts,  which  do  not,  however,  neces- 
sarily advance  equally.  In  the  lower  forms  there  is 
no  clear  distinction  between  the  sexual  and  non-sexual 
plants,  but  in  the  highest  green  algse  this  becomes 
recognizable,  but  is  most  clearly  seen  in  the  Archegoni- 
ates,  where  the  alternation  of  generations  is  very  con- 
spicuous. In  the  lower  Archegoniates  the  sexual  phase, 
or  gametophyte,  is  the  more  important,  but  in  the  higher 
ones  the  sporophyte  becomes  more  and  more  prominent 
until,  in  the  seed-bearing  plants,  the  gametophyte  is 
exceedingly  rudimentary  and  may  be  reduced  to  a  very 
few  cells  and  is  never  capable  of  independent  growth. 

The  angiospermous  flowering  plants  are  the  most 
modern  and  specialized  members  of  the  vegetable 
kingdom,  and  have  largely  superseded  the  earlier  plant 
types,  although  remnants  of  the  latter  persist,  espe- 
cially among  aquatic  forms,  which  have  been  subjected 
to  less  marked  changes  of  environment  and  less  keen 
competition  in  the  struggle  for  existence. 


INDEX 


Acacia,  261,  271,  274. 

Accessory  reproductive  parts,  28. 

Aceraceae  (Maple  family),  209. 

Aconitum,  208,  247. 

Acrid  protective  substances,  260. 

Actinomorphic  flowers,  218. 

Adder-tongue  (Erythronium) ,  189, 
Fig.  46. 

Adder-tongue  fern  (Ophioglossum) , 
127,  132,  133,  134,  Fig.  34. 

Adiantum,  234. 

/Ecidiomycetes  (see  also  "Rusts"), 
89,  90,  94,  95,  99 ;  hetercecism  of, 
88,  89 ;  Fig.  23. 

^cidium,  89,  Fig.  23. 

Africa,  231. 

Agave  (see  also  "  Century  Plant  "), 
237,  272. 

Aggregates,  215,  219,  Fig.  53. 

Air-plants  (see  also  "Epiphytes"), 
183,  195,  276. 

Alaska,  239. 

Alcoholic  fermentation,  96. 

Alder,  240. 

Alfilaria  (Erodium  cicutarium) ,  241. 

Algae,  16,  17,  19,  20,  22,  24,  27,  28,  43, 
48,  49,  61,  63,  84,  85,  86,  97,  98,  99, 
101,  103,  105, 106,  109, 112,  119, 126, 
221,  222,  228,  265,  280,  284;  calca- 
reous A.,  14,  222;  classification  of 
A.,  49  ;  coralline  A.,  14,  222  ;  fossil 
A.,  221,  222;  marine  A.,  20,  63. 

Algae,  Brown  (see  also  "  Brown 
Algse,"  "Phaeophycese"),  27,  49, 
63,  64,  71,  73,  76,  78,  79,  263,  287, 
295. 


Algae,  Green  (see  also  "  Green  Algae," 
"Chlorophyceae"),  44,  46,  47,  49, 
50,  55,  59,  63,  64,  65,  68,  70,  71,  74, 
75,  76,  78,  79,  83,  99,  100,  104,  106, 
118,  129,  221,  286,  287,  288,  289,  296, 
298. 

Algae,  Red  (see  also  "Red  Algae," 
"Rhodophyceae"),  49,  63,  70,  72, 
73,  74,  78,  79,  222,  263,  265,  287, 
295. 

Alga-fungi  (see  also  "  Phycomy- 
cetes"),  81,  288. 

Alisma,  207. 

Alpine  floras,  231. 

Alps,  236. 

Alternation  of  generations,  104,  105, 
266,  289,  298. 

Amaryllis,  A.  family,  191. 

Ament  (catkin),  206. 

Amentacese,  206,  217,  219,  229,  Fig. 
49. 

Amoeba,  5,  34,  Fig.  2. 

Anabaena,  35,  264,  Fig.  5. 

Andes,  234. 

Androecium  (see  also  "Stamen"), 
179. 

Anemone,  208,  246;  A.  coronaria, 
246,  Fig.  55. 

Anemophily,  anemophilous  flowers, 
244. 

Aneura,  126. 

Angiospermae,  Angiosperms,  155, 
156,  157,  159,  161, 167, 175,  176, 177, 
178,  179,  180,  182,  183,  196,  199, 
200,  209,  218,  226,  228,  238,  244,294, 
295,  298;  classification  of,  183; 
flowers  of,  178;  fossil  A.,  gameto- 
phyte  of,  178,  179 ;  pollination  of, 


299 


300 


INDEX 


181;    reproductive  parts  of,   180, 

181 ;  sporophyte  of,  181,  182. 
Animal  parasites,  88,  89. 
Animals,  10,  11,  24,  25,  26,  27,  30,  34, 

229,  241,  243,  257,  259,  260. 
Anisocarpse,  213,  214,  215,  219,  Figs. 

52,  53. 

Annularieae,  224. 
Annulus   (of  fern-sporangium),  29, 

135,  Fig.  35. 
Antarctic  flora,  231. 
Anther,  142,  179,  194,  215,  252,  283. 
Antheridium,  53,  (52,  63,  74,  83,  84, 

91,  92,  103,  116,  128,  129,  145,  150, 

151,  160,  163,  164,   170,   180,   181; 

Angiosperms,  180, 181 ;  Archegoni- 

ates,    103;    Ascomycetes,    91,  92; 

Chara,  62,  63;   Confer  vaceae,  76; 

Cycas,  163;  Cystopus,  84;  Ferns, 

128,  129;  Isoetes,  150;   Pine,  170; 

Red  Algae,  74;  Riccia,  103;   Sela- 

ginella,    145;    Water-mould,    83; 

Figs.  9,  12,  14,  20,  21,  24,  26,  33,  38, 

39,  40,  42. 
Anthoceros,  109,   112,  113,  115,  116, 

118,  120, 121, 122,  123, 124,  132,280, 

290,  Figs.  28,  31. 
Anthocerotaceae,  125,  128,  132. 
Antlmrium,  186,  247. 
Antipodal  cells,  179,  180,  202,   Fig. 

44. 

Ants,  260,  261. 
Apetalse,  217. 
Apical  cell,  61,  62,  106,  126,  131,  140, 

170;    Chara,  61,   62;    Equisetum, 

140;   Fern,   126;  Pine,  170;   Figs. 

14,  32,  36,  42. 
Apocarpae     (Monocotyledons),     185, 

186,   197,  217,  Fig.  45;   (Dicotyle- 
dons), 217. 
Apocynaceae,  215. 
Apophysis,  117,  120,  Fig.  30. 
Appalachian  Mountains,  232. 
Apple,  182. 

Aquatic  plants,  17,  18,  269,  270,  295. 
Aquilegia  (see  also  "Columbine"), 

207,  208,  247,  Fig.  50. 
Aracese,  186. 
Aralia,  Arahacese,  211,  212. 


Arborescent  Liliacese,  191,  192. 
Arborescent  Monocotyledons,  268. 
Arbor- vitae  (Thuja),  168. 
Arbutus,  214,  240. 
Archegoniatae,    Archegoniates,    16, 

102, 103,  104, 105,  160,  164,  180,  266, 

289,  298. 
Archegonium,  102,  103,  108,  110,  111, 

116,  118,  128,  129, 145,  148, 150, 151, 

159,  160,  163, 164,  169, 170, 180,  289, 

292,  Figs.  26,  33,  38,  39,  40,  42. 
Archesporium,  105,  109, 125, 132, 134, 

135. 

Archicarp,  91,  92,  Fig.  24. 
Arctostaphylos   (see   also  "Manza- 

nita"),274. 
Arethusa,  194,  Fig.  47. 
Aril,  165. 

Arisaema,  185,  186,  Fig.  45. 
Arisarum,  85. 
Arizona,  237,  271. 
Aroid,  Aroideee,  185,  186,  187,  197, 

198,  206,  217,  219,  234,  244,  247,  249, 

260,  278,  Fig.  45. 
Arrow-head   (Sagittaria) ,  185,   Fig. 

45. 

Arum,  186. 
Asclepias,  Asclepiadacese   (see  also 

"Milkweed"),  215,  254,  255,  Fig. 

57. 

Ascobolus,  91,  Fig.  24. 
Ascomycetes    (see    also    "  Sac-fun- 
gi "),  90,  91,  92,  93,  95,  96,  97,  98, 

99,  100,  280,  Fig.  24. 
Ascospore,  91,  92. 
Ash,  215,  295. 
Aspen,  230. 

Aspidium,  135,  Fig.  35. 
Assimilation  (see  also  "  Carbon-as- 
similation,"   "  Photo-synthesis  "), 

10,  17,  24,  25. 
Assimilative   tissues,   10,    107,    112, 

116,  117,  120. 
Aster,  216. 
Atlantic  North  America,- Flora  of, 

234,  235,  237,  238. 
Australia,  147,  271. 
Auxiliary  cells  (of  Rhodophyceae) , 

74,  75,  78. 


INDEX 


301 


Auxospores  (of  Diatoms) ,  65. 

Awn,  243,  283,  Fig.  54. 

Azalea,  213,   214;   A.   viscosa,   213, 

Fig.  52. 
Azolla,  280. 


B 


Bacillus,  35;  B.  typhi,  35,  Fig.  5; 

B.  tetani,  35,  Fig.  5. 
Bacteria      (see     also     "  Schizomy- 

cetes  "),  3,  8,  17,  18,  34,  35,  36,  37, 

38,  45,  46,  96,  242,  258,  284. 
Bald  cypress  ( Taxodium  distichum) , 

168,  227. 
Bamboo,  188. 

Banana,  B.  family,  192,  193,  240. 
Bangiaceae,  71. 
Barberry,  88,  90,  252. 
Barbs  (of  fruits  and  seeds),  243. 
Basidiomycetes,  90,  93,  94,  95,  96,  97, 

98,  99,  100,  Fig.  25. 
Basidium,  93,  94,  Fig.  25. 
Bast  (see  also  "  Phloem  "),  124. 
Batrachospermum,  74,  Fig.  20. 
Bay-tree  (Umbellularia) ,  240. 
Bean,  212,  283. 
Beech,  237,  238. 
Bees,  249,  252,  254. 
Beggar 's-ticks       (Echinospermum) , 

243. 

Beggiatoa,  35,  Fig.  5. 
Bignonia,  214. 
Biology,  11. 
Birch,  207,  228,  230. 
Birds,  243,  249. 
Bird's-nest    fungus    (Cyathus),  94, 

Fig.  25. 

Bitter-sweet  (Celastrus),  237. 
Black-fungi   (see  also   "Pyrenomy- 

cetes"),87,  93. 
Black  knot  (Plowrightia  morbosa), 

93. 
Black-mould     (see    also     "  Mucor, 

Mucorini  "),  85,  86,  Fig.  22. 
Bladder-kelp  (see  also  "  Macrocys- 

tis,"  "  Nereocystis  "),  68. 
Bladder-weed     (see    also    "  Utricu- 

laria"),  108,  204,  258,  259,  Fig.  58. 


Blasia,  106,  279,  Fig.  27. 
Blood-root  (Sanguinaria),  200. 
Blue-green  Algae    (see  "  Cyanophy- 

ceae,"  "  Schizophyceae  ") . 
Blue-gum  (see  "Eucalyptus"). 
Blue-mould  (Penicillium),  92. 
Borraginacese,  243,  295. 
Botany,   a  department  of   Biology, 

11. 
Botrychium,  133,  134;   B.  simplex, 

134;    B.    Virginianum,    133,   134, 

Fig.  34. 

Bract,  185,  193,  195,  246,  247,  248. 
Brake  (Pteris  aquilina),  136. 
Bramble,  232,  240. 
Breadfruit,  240. 

Bromeliaceas,  194,  198,  231,  276. 
Broom     (Sarothamnus) ,    250,    252, 

Fig.  56. 
Brown  Algae   (see  also  "  Phaeophy- 

ceae  "),  27,  49,  63,  64,  71,  73,  76,  78, 

79,  263,  288,  295. 
Brown  Flagellates  (Dinoflagellata) , 

64,  76,  288. 
Bryineae,  115. 
Bryophyta,    Bryophytes    (see    also 

"  Mosses  "),  16,  101,  113,  118,  122, 

128, 130,  131,  153,  223,  273,  287,  289, 

290,  Figs.  27,  28,  29,  30,  31. 
Buckwheat  family   (Polygonaceae) , 

208. 
Buds  (see  also  "  Gemmae  "),  102, 126, 

270,  281. 
Budding    in    animals    and    plants, 

25. 

Bulb,  189,  192,  201,  273,  282. 
Bumblebees,  247,  251. 
Bur-clover  (Medicago  denticulata) , 

241,  243,  Fig.  54. 
Burdock,  216,  243. 
Bur-marigold  (Bidens),  241. 
Bur-reed  (Sparganium) ,  186,  198. 
Burs,  243. 
Buttercup    (Ranunculus),    207,  208, 

246,  247,  Figs.  50,  59. 
Buttercup  family    (Ranunculaceae), 

186,  207,  208,  232,  247. 
Butterflies,  247,  249,  255. 
Butterwort  (Pinguicula) ,  259. 


302 


INDEX 


Cactaceae,  211,  212. 

Cacti,  18,  204,  231,  237,  260,  261,  272. 

Caesalpinese,  212. 

Cakile  (Sea-rocket),  275. 

Calamites,  Calamitese,  142,  224,  225. 

Calcium,  72. 

Calcium  carbonate,  61,  72. 

California,   167,   174,  187,   192,  237, 

238,  239,  241. 
Calla-lily   (Richardia),  25,  185,  246, 

247,  Fig.  55. 
Callitkanmion,  72 ;  C.  floccosum,  72, 

Fig.  19. 

Calochortus  (see  "  Mariposa-lily  "). 
Calyciflorae,  211,  212,  218,  219. 
Calyptra,  110. 
Calyx,   178,  179,  182,  190,  205,  206, 

209,  210,  211,  215. 
Cambium,  172,  200,  268. 
Canada,  230. 

Canada  thistle,  215,  Fig.  53. 
Caniia,  Canna  family,  192,  193,  194, 

249,  Fig.  47. 

Cape  region  (of  Africa),  192. 
Caprif  oliaceae  (Honeysuckle  family) , 

215. 

Capsella,  84,  200,  Fig.  48. 
Capsule  (of  Mosses),  111,  116,  117, 

120,  Fig.  30. 

Carbo-hydrate,  8,  11,  19. 
Carbon,  2,  8,  9,  19,  30,  80,  297. 
Carbon      assimilation      (see      also 

"  Photo-synthesis  ") ,  10,  11,  17, 22, 

24,  37,  64,  71,  80,  263,  278. 
Carbon  dioxide,  8, 11,  17,  19,  21,  30, 

262,  269,  297. 
Carboniferous  formations,  138,  142, 

147,  154,  165,  174,  175,  223,  224,  225, 

226. 
Cardinal  flower  (Lobelia  cardinalis), 

249. 

Carnation  (Dianthus),  248. 
Carnivorous  plants,  257,  258,  259. 
Carpel,  159,  160,  161,  169,  177,  179, 

181,  184,  185,  189,  190,  196, 197,  202, 

205,  206,  207,  208,  209,  210,  211,  213, 

217,  218,  245,  293. 
Carpogonium,  73,  74. 


Carpospore,  74. 

Caryophyllaceae    (see    "Pink    fam- 
ily"). 
Castor-bean     (Ricinus),     200,     210, 

Fig.  48. 
Catalpa,  214. 

Catch-fly  (Silene),  206,  248,  Fig.  49. 
Catkin,  20(5. 

Cat-tail  rushes  (Typhaceae),  186. 
Caulerpa,  56,  58;   (7.  plumaris,  56, 

Fig.  11. 

Ceanothus,  240. 
Cecropia,  261. 
Cedar-apple,     Cedar-rust     (Gymno- 

sporangium),  89,  90,  95,  Fig.  23. 
Celastrus,  237. 

Cell-division,  7,  8,  35,  40,  41,  285. 
Cell-membrane     (see     also     "Cell- 
wall"),  4,  284. 
Cell-plasm  (Cytoplasm),  4,  6. 
Cells,  4,  284. 
Cell-sap,  7. 
Cell-wall,  4,  284. 
Cellulose,  11. 
Centrosome,  7. 
Centrospermse,    206,    208,   217,   218, 

219,  Fig.  49. 

Century  plant  (Agave) ,  272. 
Cercis,  212. 
Cereals,  188. 
Chaetophora,  51. 
Chaparral,  240. 
Chara,  22,  62,  63,  75 ;  C.  crinita,  75 ; 

Fig.  14. 
Characefe,  14,  61,  62,  63,  78,  79,  103, 

118,  222. 

Cheiranthus,  210,  Fig.  51. 
Chenopodiacese   (Pig-weed  family), 

208. 

Cherry,  93,  165. 
China,  166,  236. 
Chlamydomonas,  42. 
Chlorophyll,  6,  8,  12,  20,  22,  23,  48, 

49,  80,  196,  262,  263,  269,  278,  297. 
Chlorophyceae     (see     also    "Green 

AlgEe"),  46,  47,  49,  70,  78,  79,  101, 

286,  288. 
Chloroplast    (see    also    "Chromato- 

phore"),  6,  19,  20,  21,  38,  59,  60, 


INDEX 


303 


64,  65,  118 ;  of  Brown  Algae,  64, 65 ; 
of  Conjugates,  59,  60;  of  Volvo- 
cineae,  38. 

Cholera  bacillus  (Microspira  com- 
ma), 35,  Fig.  5. 

Chondrus  (see  also  "Irish  Moss"), 
71,263;  C.  crisp w*,  263. 

Choripetalae,  205,  206,  207,  208,  209, 
210,  213,  217,  219,  229,  Figs.  44, 
50,  51. 

Chromatin,  5,  6. 

Chromatophore,  6,  8,  19,  20,  21,  38, 
59,  60,  64,  65,  112,  118. 

Chromosome,  6. 

Cilia,  34,  35,  38,  39,  46,  57,  67. 

Cinnamon-fern  ( Osmunda  cinna- 
momed),  134. 

Cirsium,  215. 

Cistiflorae,  209. 

Cladophora,  51,  52,  Fig.  8. 

Classification,  12,  15,  16. 

Clematis,  208. 

Climate  (as  a  factor  in  distribu- 
tion), 229, 235. 

Climbing  fern  (Lygodium),  135, 
Fig.  35. 

Climbing  plants,  23,  186,  202,  277, 
278,  297. 

Climbing  stems,  201,  202,  278. 

Closterium,  59,  Fig.  13. 

Clover,  24,  283. 

Club-mosses  (see  also  "  Lycopodi- 
nese,"  "  Lycopods  "),  128,  143,  144, 
145,  147,  153,  154,  161,  167,  232, 
273,  291,  Figs.  37,  38. 

Cluster-cup  (^cidium),  88,  89,  90, 
Fig.  23. 

Coal-measures  (see  also  "Carbon- 
iferous"), 173,223,226. 

Cockspur-thorn  (Cratsegus  crus- 
galli},W,  Fig.  23. 

Cocoanut,  240. 

Cold  (effect  on  plants),  18,  281. 

Coleochaete,  52,  54,  55,  77,  101,  109, 
110,  112,  286,  289;  C.  pulvinata, 
54;  C.  scutata,  54;  Fig.  10. 

Color  of  flowers,  295. 

Columbine  (Aquilegia),  207,  208,  247, 
Fig.  50. 


Columella,  86,  113,  116,  117, 124, 135; 

in  Anthoceros,  113, 124 ;  in  Mosses, 

117,  118;  in  Mucor,  86. 
Column    (Gynostemium),    194,   195, 

254,  256. 
Compositae,  196,  215,  216,  218,  219, 

231,  238,  253. 
Compound  leaf,  187,  201. 
Compound  pistil,  197,  209,  213. 
Conditions  of  plant  life,  17,  262. 
Conducting  tissue  (of  pistil),  181. 
Cone   (see  also    "  Strobilus "),   140, 

142,  143,  146,  164,  168,  1(59,  170;  of 
Conifers,  168,  169;  of  Cycads,  163; 
of  Equisetum,  140;   of  Lycopods, 

143,  145. 
Conferva,  286. 

Confervaceje,  27,  51,  52, 53, 54,  55, 58, 
60,  67,  71,  98,  106,  288,  289;  repro- 
duction of,  52,  53 ;  Figs.  8,  9,  10. 

Conidium,  82. 

Conifers,  Conifers,  147, 156, 157, 161, 
165,  166,  167,  168,  169, 170,  171,  172, 
173,  174,  175,  181,  226,  227,  239,  267, 
281,  293;  fossil  C.,  173,  226,  227; 
structure  of  C.,  167,  168,  169,  170, 
171,  172;  Figs.  41,42. 

Conjugatae,  55,  59,  78,  79,  86;  struct- 
ure of  C.,  59,  60;  Fig.  13. 

Conjugation,  in  Mucor,  86 ;  in  Spiro- 
gyra,  61. 

Conocephalus,  106,  Fig.  27. 

Contortae,  215. 

Contractile  vacuole,  39,  50. 

Corals,  25,  72. 

Coral  honeysuckle  (Lonicera  semper- 
virens),  249. 

Coral  reefs  (due  to  calcareous  algae), 
72. 

Coralline  algae  (Corallineae),  14,  72, 
222. 

Cordaitese,  174,  226. 

Cornaceae  (Dogwood  family) ,  212. 

Cornus  (see  also  "Dogwood"),  246, 
247 ;  C.  florida,  246,  247,  Fig.  55. 

Corolla,  178,  179,  186,  212,  213,  215, 
216,  218,  244,  245. 

Cosmarium,  4,  59,  Figs.  1,  13. 

Cotton  wood  (Populus),  240. 


304 


INDEX 


Cotyledon,  130, 131, 145, 170, 171, 182, 

184,  185,  199,  200,  204. 
Crab-apple,  89. 
Cranberry,  214. 
Creepers      (see      also      "  Climbing 

Plants,"  "Liana"),  276. 
Cretaceous  formations,  227,  228. 
Crimson  balm  (Monarda  didyma), 

249. 
Crimson  currant  (Ribes  speciosum) , 

249. 
Cross-fertilization,  195,  208,  214,  244, 

245,  246,  247,  249,  250,  251,  252,  253, 

254,  255,  256. 
Croton,  210. 

Cruciferae  (Mustard  family),  209. 
Cruciflorae,  209. 
Crustacea,  108. 
Cup-fungi    (Ascobolus,  Peziza),  91, 

92,  Fig.  24. 
Cupuliferae,  281. 
Currents   (factors   in  distribution), 

229. 

Cuscuta  (see  also  "Dodder"),  279. 
Cuticle,  269. 
Cutleria,  70. 
Cyanophyceae    (see    also    "  Schizo- 

phycese"),  36,  37,  98. 
Cyathus,  94,  Fig.  25. 
Cycads,  Cycadaceas,  155, 156, 157, 161, 

162,  163,  165,  1(36,  167, 168, 169, 175, 

176, 196,  226, 292,  293 ;  structure  of, 

162, 163, 164, 165 ;  fossil  C.,  165, 226 ; 

Fig.  40, 
Cycas,  160, 161, 162, 163, 166, 168, 169 ; 

C.    circinalis,    163,    Fig.   40;    C. 

revoluta,  162, 163,  Fig.  40. 
Cyclantherae,  187. 
Cynoglossum,  243,  Fig.  54. 
Cyperaceae,  188. 
Cypress,  168. 

Cypripedium,  194,  234,  Fig.  47. 
Cystocarp  (of  Red  Algse),  72,   Fig. 

19. 
Cystopus  (see  also  "White-rust"), 

82 ;  C.  candidus,  82,  Fig.  21. 
Cytology,  7, 
Cytoplasm,  4,  6. 


D 


Dactylis,  185,  Fig.  45. 

Daisy,  216. 

Dandelion,  91,  215,  216,  241,  Fig.  53. 

Darwin,  257. 

Dead-nettle  (Lamium),  215,  Fig.  53. 

Deciduous  trees,  238. 

Decomposition  (due  to  bacteria),  35, 

36. 
Delphinium  (see  also  "Larkspur"), 

207,  208,  247,  Fig.  50. 
Desert     plants    (see    also    "  Xero- 

phytes  "),  18,  204,  260,  271. 
Deserts,  229,  233,  234,  237,  271,  297. 
Desmid,  Desmidiaceae,  8,  26,  59,  60, 

85,  Figs.  1,  13. 

Devonian  formations,  147,  223,  225. 
Diatomin,  65. 
Diatoms,  Diatomaceae,  2,  14,  65,  66, 

222,  Fig.  15. 
Diclinous  flowers,  206. 
Dicotyledons,  141,  162,  168,  173,  183, 

187,  190,  196,  199,  200,  201,  202,  203, 

204,  205,  206,  212,  217,  218,  219,  228, 
229,  267,  268,  269 ;  classification  of, 

205,  212;    embryo    ofj    199,    203; 
flowers  of,  206,  216;  structure  of, 
200,  201,  202. 

Digitalis  (see  also  "  Foxglove  "), 253. 

Dinoflagellata,  76. 

Diouaea  (see  also  "  Venus's  Fly- 
trap"), 257. 

Diospyrinae  (Persimmon  family), 
214. 

Disease  germs,  36. 

Distribution  of  plants,  220,  229. 

Dock  (Rumex),  241. 

Dodder  (Cuscuta),  12,  205,  278,  279, 
288. 

Dodecatheon,  213,  Fig.  52. 

Dogbane  family  (Apocynaceae),  215. 

Dogwood  family  (see  also  "Cornus," 
"Coruaceaa"),  212,  240,  246,  247, 
Fig.  55. 

Dracaena,  191. 

Drosera  (see  also  "  Sundew  "),  257, 
258 ;  D.  longifolia,  258,  Fig.  58. 

Drought  (effect  on  plants),  273,  274> 


INDEX 


305 


Duckweed   (Lemna),  182,   185,  187, 
Fig.  45. 


E 


Eagle-fern  (see  Pteris  aquilina). 

Earth-star  (Geaster) ,  94,  Fig.  25. 

Eastern  Asia,  232,  233,  235. 

Eastern  America,  232,  233,  235. 

Ectocarpus,  67,  68;  E.  granulosus, 
67,  Fig.  16;  E.  siliculosus,  67, 
Fig.  16. 

Eel-grass  (Vallisneria) ,  22. 

Egg,  egg-cell,  27,  39,  41,  53,  54,  58, 
63,  69,  74,  102,  103,  108,  109,  128, 
129,  130,  165,  171,  179,  181;  Chara, 
63;  Conifers,  171;  Ferns,  129; 
Fucus,  69;  Ked  Algae,  74;  Riccia, 
103 ;  Vaucheria,  58. 

Egg  apparatus,  179,  180,  181,  202, 
Fig.  44. 

Elater,  29,  109,  111,  140,  142,  Figs. 
28,  36. 

Electricity,  24. 

Elm,  237,  238. 

Embryo,  15,  109,  116,  129,  130,  131, 
137,  140, 144, 145,  158,  159,  170,  171, 
172, 182, 183,  184, 185,  200,  203,  217 ; 
Conifers,  170,  171,  172;  Dicotyle- 
dons, 200;  Equisetum,140;  Ferns, 
129, 130 ;  Lycopods,  144, 145  ;  Mono- 
cotyledons, 185,  Figs.  28,  33,.  45, 
48. 

Embryo-sac,  145,  159,  165,  178,  179, 
180,  Figs.  43,  44. 

Endosperm,  150,  152,  165,  171,  172, 
179,  180. 

Energid,  4. 

Entomophily  (see  "  Insects  "). 

Entomophthoracese  (see  also  "  In- 
sect-fungi "),  86. 

Environment  (effect  of) ,  262. 

Eopteris,  223. 

Ephedra,  173. 

Epidermis,    epidermal    tissues,    21, 

124,  141,  267. 

Epiphytes,  194, 195,  275,  276,  277,  297. 

Equisetinese  (see  also  "  Horse-tail "), 

128,  139,  140,  143, 147, 153, 154,  155, 


157,  224,  fossil  E.,  143,  154;  hete- 
rospory, 155. 

Equisetum  (see  also  "  Horse-tail," 
"Scouring-rush"),  139,  140,  141, 
142,  143,  144,  146,  161,  168,  224; 
fossil,  142,  224 ;  structure,  139,  140, 
141,  142;  E.  maximum,  140,  Fig. 
36 ;  E.  pratense,  140,  Fig.  36. 

Ergot  (Claviceps purpurea) ,  93. 

Erodium  (see  also  "  Alfilaria  "),  250, 
Fig.  56. 

Erythronium,  189,  Fig.  46. 

Eschscholtzia,  240.- 

Essential  oils,  260. 

Eucalyptus,  274,  275. 

Eucyclse,  209. 

Euphorbia,  Euphorbiacese,  210. 

Eurotium,  92. 

Eusporangiatse,  127,  128,  133,  134, 
135, 136, 137,  138,  139, 144,  149, 151, 
154,  155, 156, 157,  223,  224 ;  embryo 
of,  130;  fossil  E.,  138,  Fig.  34. 

Evening  primrose  (CEiiothera) ,  211, 
238. 

Evergreens,  166. 

Exogens  (see  "Dicotyledons"). 

Eye-spot,  39,  50,  52. 


F 


Fan-palm,  187. 

Ferns  (see  also  "  Pteridophytes," 
"  Filicineaa  ") ,  15,  16,  27, 28,  29,  55, 
102,  112,  116, 121,  123, 126, 128, 129, 
130,  131,  132, 133,  i:34,  135, 136,  137, 
138, 139,  140,  146,  148, 149,  153, 154, 
155,  156,  157, 162, 163,  164,  166,  167, 
196,  201,  220,  223,  224,  267,  273,  275, 
276,  285 ;  embryo  of,  129, 130 ;  fossil 
F.,  154,  220,  223,  224 ;  gametophyte 
of,  125,  126,  127 ;  heterospory,  148, 
149,  150;  sporophyte,  122,  323. 

Fertilization,  41,  54,  58,  CO,  63,  70, 
74,  75,  83,  85,  91,  103,  104,  108,  119, 
128,  160,  163,  164, 169, 170,  181,  203, 
266,  in  Archegoniates,  103,  104;  in 
Ascomycetes,  91 ;  in  Characese,  63, 
in  Conifers,  169,  170;  in  Cycads, 


306 


INDEX 


164 ;  in  Cystopus,  85 ;  in  CEdogo- 
nium,  53,  54 ;  in  Phaeophyceae,  70 ; 
in  Rhodophyceae,  74,  75 ;  in  Sapro- 
legnia,  83;  in  Spermatophytes, 
160 ;  in  Vaucheria,  58. 

Fig  (Ficus) ,  182. 

Figworts  (Scrophulariaceae) ,  214, 
218,  219. 

Filices,  136. 

Filicineae  (see  also  "Ferns"),  157. 

Filmy  ferns  (see  also  "  Hymenophyl- 
laceae),  126,  127,  131,  135,  151,  276, 
Fig.  35. 

Fimbriaria  Californica,  103,  Fig. 
26. 

Fir,  169,  227,  230,  245. 

Fish,  259. 

Fission,  7,  25,  35,  36,  65,  285. 

Fission  plants  (Schizophyta) ,  34. 

Fission  algae  (see  "  Schizophyceae," 
"  Cyanophycese"). 

Flagellate  Infusoria,  Flagellata,  33, 
38,  45,  64,  76,  284,  288. 

Floats  (of  kelps),  68,  69,  Figs.  17, 
18. 

Floral  axis,  182,  208,  209,  211. 

Floral  envelope  (see  also  "Peri- 
anth'"'), 178, 

Floral  leaves,  161,  245. 

Florida,  162. 

Flower,  10,  14,  160,  161, 165, 168, 170, 
174,  177,  178,  182, 183, 184, 185, 186, 
187,  188,  189, 190, 191, 192, 193,  194, 
195,  196,  197,  202,  206,  207,  208,  209, 
210,  212,  213,  214,  215,  216;  of 
Angiosperms,  Figs.  43,  44,  45,  46, 
47,  48,  49,  50,  51,  52,  53;  of  Coni- 
fers, 165,  170,  Figs.  41,  42;  of 
Cycads,  162,  163,  Fig.  40. 

Flowering  plants  (see  also  "Seed 
Plants,"  "Spermatophytes"),  9, 
16,  19,  21,  28,  29,  62,  137,  141,  142, 
144,  145,  150,  152, 155, 158, 161,  275, 
276,  288,  293,  298. 

Food  of  plants,  19,  171,  192,  217,  262. 

Foot  (of  embryo),  109,  111,  120,  129, 
130. 

Forests,  238,  240,  241,  276,  277. 

Fossil  plants,  13,  14,  66,  72,  138,  142, 


143,  147,  154,  155,  165,  173,  174, 
175,  217,  220,  221,  222,  223,  224,  225, 
226,  227,  228,  229;  Alga?,  72,  221, 
222;  Conifers,  173,  174,  226,  227, 
228;  Cycads,  165,  226;  Diatoms, 
66,  222;  Dicotyledons,  217,  227; 
EquisetinefE,  142, 143, 147, 155,  224, 
225;  Ferns,  154,  220,  223,  224; 
Fungi,  222  ;  Gingko,  166,  226,  227 ; 
Lycopods,  147,  154,  155,  225,  226; 
Mosses,  223. 

Fossombronia  longiseta,  109,  Fig. 
28. 

Foxglove  (Digitalis) ,  253. 

Fox-grape  (Vitis  labrusca),  237. 

Fragmentation  of  nucleus,  62. 

Fresh-water  algae,  264,  265,  287,  289. 

Fritillaria,  239. 

Fruit,  29,  165,  181,  182,  183,  202,  211, 
216,  243. 

Fucaceae,  69,  Fig.  18. 
j  Fuchsia,  210,  211,  249,  Fig.  51. 

Fucus,  69;  F.  vesiculosus,  69,  Fig. 
18. 

Funaria,  114,  117,  Figs.  29,  30. 

Fungi,  11,  16,  28,  33,  48,  73,  80,  81, 
82,  83,  84,  86,  87,  91,  92,  93,  94,  95, 
96,  97,  98,  99, 100, 101, 181,  222,  242, 
279, 288, 297  ;  alga  F.  ( see  "  Phyco- 
mycetes") ;  insect  F.  (see "Insect- 
fungi");  fossil  F.,  222;  true  F. 
(see  "  Mycomycetes  "). 


G 


Gamete,  26,  39,  44,  52,  53,  58,  67, 
285,  287;  Confervaceae,  52,  53; 
Phaeophyceae,  67,  70;  Protococca- 
cese,  44;  Siphoneae,  58;  Volvoca- 
ceae,  39. 

Gametophore,  114. 

Gametophyte,  55,  105,  106,  110,  111, 
112,  113,  114,  115,  118,  122,  123,  126, 
127, 128, 129,  131,  137,  139,  144, 145, 
147,  149,  150,  151, 152, 153, 156,  158, 
159, 160, 163, 165,  168,  169,  171,  178, 
180,  182,  188,  290,  291,  292,  298; 
Angiosperms,  178,  179 ;  Anthoce- 


INDEX 


307 


ros,  112,  131;  Archegoniates,  105, 
106;  Conifers,  163,  164,  168,  169; 
Cycads,  163,  164;  Equisetum,  139, 
140;  Ferns,  123,  126,  127;  Hepati- 
cae,  106,  107;  Hydropterides,  150, 
151;  Hymenophyllaceae,  126,  127; 
Isoetes,  149,  150;  Lycopods,  144, 
145 ;  Marattiaceae,  123, 127 ;  Mosses, 
114,  115,  116;  Vittaria,  126,  127, 
Figs.  27,  29,  31,  32,  40,  42,  44. 
Gamopetalae  (see  also  "  Sympeta- 

l8B"),212. 

Gasteromycetes,  94,  Fig.  25. 

Geaster,  94,  Fig.  25. 

Gemmae  (see  also  " Buds"),  102, 107, 
127. 

Gentian,  215. 

Geographical  distribution,  221,  229. 

Geological  distribution,  221. 

Geological  record  (see  also  "Fos- 
sil plants"),  220,221,  235. 

Geotropism,  24. 

Geranium,  G.  family,  209,  210,  250. 

Gerardia,  85. 

Giant  tree  (Sequoia  gigantea),  174, 
227. 

Giant  kelp  (Macrocystis,  Nereo- 
cystis) ,  63,  287. 

Gigartina.  72 ;  G.  spinosa,  72,  Fig.  19. 

Gills  (of  mushroom) ,  94,  Fig.  25. 

Ginger,  G.  family  (Zingiberaceae) , 
192,  193,  198. 

Gingko,  160,  165,  166,  175,  226, 
Fig.  41. 

Ginseng,  212. 

Glacial  epoch,  235. 

Gladiolus,  191,  273. 

Gleditschia  (see  also  "Honey- 
locust"),  212. 

Gleichenia,  136. 

Gloeotrichia,  35,  Fig.  5. 

Glumaceae,  188. 

Glyptostrobus,  227. 

Gnetaceae,  161,  173,  175. 

Gold-back  fern  (Gymno  gramme 
triangularis) ,  273. 

Gonidium  (of  Volvox),  41. 

Gramineae  (see  also  "Grasses"), 
188,  198. 


Grasses,  180,  185,  188,  189,  197,  198, 
240,  241,  259,  Figs.  45,  54. 

Gravity  (influencing  movement), 
23,  24. 

Gray,  Asa,  236. 

Green  Algae  (see  also  "  Chlorophy- 
ceae"),  44,  46,  47,  49,  50,  55,  59, 
63,  64,  65,  68,  70,  71,  74,  75,  76,  78, 
79,  83,  99,  100,  104,  106,  118,  129, 
221,  286,  287,  289,  295,  298. 

Green  monads  (see  also  "  Volvoca- 

CC83"),25. 

Greenland,  230. 

Ground-tissue,  125. 

Guava,  211. 

Gulf  States,  Gulf  of  Mexico,  187,  238. 

Gulf  Stream,  233. 

Gulf-weed  (see  also  "  Sargassum  "), 
20,  67,  69,  Fig.  18. 

Gum  trees  (Nyssa,  Liquidambar) , 
238;  (Eucalyptus),  274. 

Gymnogramme,  273. 

Gymnosperms,  Gymnospermae,  155, 
157,  158,  161,  1(52,  163,  168, 169, 171, 
173, 175,  176,  177,  178,  179,  180, 181, 
191,  226,  227,  268,  293. 

Gymnosporangium,  89,  Fig.  23. 

Gynandrae,  192. 

Gyncecium  (see  also  "  Carpel,"  "  Pis- 
til"), 179,  190. 

Gynostemium  (see  also  "  Column  "), 
254,  256. 


H 


Haeckel,  31. 

Hairs,  22,  125,  205,  215,  260,  271,  296. 

Haustorium    (see    also    "Sucker"), 

82,  91,  Fig.  21. 
Hawaiian  Islands,  234. 
Hawthorn,  89. 

Head  (of  Compositae),  215,  216. 
Heat,  evolution  of,  25. 
Hedera  (Ivy),  211. 
Heliconia,  193. 
Heliotropism,  22,  23. 
Hemidinium  nasittum,  65,  Fig.  15. 
Hepaticte  (see  also  "Liverworts"), 

101,  102,  103,  106,  109,  110,  111,  113.. 


308 


INDEX 


114,  115, 119,  120,  126,  127,  137, 156, 
289,  290,  292,  Figs.  26,  27,  28. 

Herbarium  mould  (Eurotium) ,  92. 

Hermaphrodite  flowers,  196. 

Heterocyst,  35. 

Hetercecism,  88,  89,  95. 

Heterosporous  Pteridophytes,  136, 
148, 149,  150,  151,  152,  155,  157, 158, 
178,  179,  225,  226,  293,  Figs.  38,  39. 

Heterospory,  148,  149,  151,  152,  156, 
226,  292. 

Hickory,  235,  238. 

Holdfast  (of  Algse),  67,  68,  69,  263, 
286. 

Homosporous  Pteridophytes,  148, 149, 
151,  153. 

Honey-locust  (Gleditschia),  212. 

Honeysuckle  (Lonicera),  215,  249. 

Hooks  (of  fruits),  29,  243,  295. 

Hop,  278. 

Hordeum  murinum,  243,  Fig.  54. 

Horse-tail  (see  also  "  Equisetum  ") , 
128,  139,  140,  224,  Fig.  36. 

Horse-chestnut,  270,  Fig.  59. 

Host,  80,  82,  88,  89,  181,  279. 

Houud's-tongue  (Cynoglossum) ,  243, 
Fig.  54. 

Huckleberry,  214. 

Humming-birds,  208,  247,  249. 

Humus  plants,  279. 

Huxley,  3. 

Hyacinth,  190. 

Hydrodictyon  (see  also  "Water- 
net"),  43, 44,  Fig.  7. 

Hydrogen,  2,  8,  19. 

Hydropterides  (see  also  "  Water- 
fern"),  150,  151,  Fig.  39. 

Hygroscopic  movements,  283. 

Hymenium,  95. 

Hymenophyllaceae  (see  also  "Filmy 
ferns"),  126,  127. 

Hypha,  87,  97,  181. 


Ice-plant  (Mesembryauthemum),275. 
Indian  corn,  269. 

Indian-pipe  (Monotropa  uniflora) ,  12. 
205,  279,  288. 


Indian  turnip  (Arisffima),  185,  Fig. 

45. 

Indusium,  135,  Fig.  35. 
Inferior   ovary,    189,    190,   191,   193, 

194,  210,  211,  215,  218,  Figs.  46,  47, 

51,  53. 

Inflorescence,  185,  193,  215,  216. 
Infusoria,  8. 
Inorganic  bodies,  2. 
Insects,  28,  99,  178,  189,  190, 194, 195, 

206,  208,  211,  242,  244,  245,  247,  254, 

294. 
Insect-fungi     (Entomophthoracese) , 

86,  242. 

Insectivorous  plants,  205,  257,  258. 
Integument  (of  ovule),  164,  165,  169, 

178,  202. 

Internal  cell-division,  7,  285. 
Internode,  61,  62,  140,  141. 
Iris,  I.  family,  189,  191,  197,  Fig.  46. 
Irish  moss  (Chondrus  crispus),  71, 

263. 
Isoetes,  135,  149,  150,  151,  152,  155, 

163,  179,  184;  /.  echinospora,  150, 

Fig.  39. 

Isocarpae,  213,  214,  218,  219,  Fig.  52. 
Isosporese  (see  "  Homosperous  Pteri- 
dophytes"). 
Ivy  (Hedera),211. 


Jamaica,  136,  232. 

Japan,  162,  166,  236. 

Joint-fir  (see  also  "  Gnetacese  "),  161. 

Juncacere  (Rush  family),  189. 

Jungermanuiaceae,  119. 


K 


Kalmia  (see  also  "Mountain  laurel"), 
253. 

Karyokinesis  (indirect  nuclear  divi- 
sion), 6,  62,  Fig.  3. 

Keel  (of  papilionaceous  flower),  250, 
252,  Fig.  56. 

Kelp,  20,  63,  67,  68,  69,  71,  78,  263, 
Figs.  17,  18. 


INDEX 


309 


Knot-grass  (Polygonum) ,  206,  Fig. 
49. 


LabiatfB  (Mint  family),  214,  253. 

Labiatiflora?,  214,  215,  218,  219,  Fig. 
53. 

Laboulbeniacese,  99. 

Lacunae  (air-spaces),  140,  141,  Fig. 
36. 

Lady's-slipper  (Cypripedium),  194, 
Fig.  47. 

Lamina  (of  leaf),  201. 

Laminaria,  68. 

Lainium,  215,  253,  254,  Fig.  53. 

Larch  (Larix) ,  168. 

Larkspur  (Delphinium),  207,  208, 
247,  Fig.  50. 

Laurel,  21,  230. 

Leaf,  10,  20,  21,  30,  69,  106,  107,  114, 
115,  126,  129,  130, 131,  140,  145, 146, 
149,  150,160,  161,  162,  163, 167,  168, 
'183,  187,  193,  196,  201,  268,  290; 
Cycads,  162,  163;  Conifers,  167, 
168;  Equisetum,  140;  Ferns,  130, 
131;  Isoetes,  150  ;  Liverworts,  106, 
107 ;  Lycopods,  144, 145 ;  Phaeophy- 
ceae,  69;  Saprophytes,  196,  279; 
Xerophytes,  271. 

Leaf-cutting  ants,  261. 

Leaf-tendrils,  278. 

Leguminosae  (Pea  family),  204,  212, 
231,  252,  280. 

Lejeunia,  106,  Fig.  27. 

Lemna  (see  also  "  Duckweed  "),  182, 
185,  187,  270,  Fig.  45. 

Lepidodendron,  147,  148,  155,  156, 
172,  226. 

Leptosporangiatae,  133,  134,  135, 136, 
137, 138,  146,  150, 154,  155,  156, 157, 
224 ;  distribution  of,  136,  137,  138 ; 
fossil,  138,  224;  Sporangium,  134, 
135,  Fig.  35. 

Liana  (see  also  "Creeper,"  "  Climb- 
ing plants"),  276. 

Lichens,  97,  98,  99,  100,  275,  276,  280. 

Life-history,  as  a  clue  to  relation- 
ships, 15. 


Light  (influence  on  plant  growth), 
17,  23,  30,  262,  277,  282,  297. 

Lilac,  215. 

Liliaceae  (see  "  Lily  family  "). 

Liliiflorse,  189,  192,  Fig.  46. 

Lily,  L.  family,  189,  190,  191,  197, 
198. 

Linaria,  215,  Fig.  53. 

Linin,  5. 

Linngea,  230,  239. 

Lip  (of  Orchids),  194,  195,  254,  255. 

Liriodendron,  207,  Fig.  50. 

Liverwort  (see  also  "  Hepaticae  "), 
29, 101,  102,  103,  106,  107,  108,  109, 
111,  112,  113, 114, 115,  116,  119,  120, 
121,  125, 126,  128,  129,  132,  137,  148, 
153,  156, 157,  223,  273,  275,  276,  283, 
289;  embryo,  109,  129;  fossil  L., 
223;  leafy  L.,  106,  107,  114,  119; 
thallose  L.,  106,  107,  112,  115,  119; 
Figs.  26,  27,  31. 

Lizard,  reproduction  of  lost  parts, 
25. 

Locomotion  in  plants,  282. 

Locust  (Robinia) ,  24,  283. 

Lodicule,  185. 

Loranthus,  278. 

Lotus  (Nelumbo),  207,  Fig.  50. 

Louisiana,  238. 

Lupine,  241,  280. 

Lycoperdon  (see  also  "Puff-ball"), 
95. 

Lycopod,  Lycopodiheae  (see  also 
"Club-moss"),  139,  143,  144,  145, 
152, 155,  156, 157,  167,  175,  225,  292, 
293;  embryo  of,  144,  145;  fossil  L., 
147, 154, 155, 225, 226 ;  gametophyte 
of,  143,  144,  145;  sporophyte,  146; 
Figs.  37,  38. 

Lycopodiaceae,  143,  144. 

Lycopodium,  143,  144,  145,  146,  147. 
148,  152,  225;  L.  clavatum,  143; 
L.  dendroideum,  143,  Fig.  37. 

Lygodium,  135,  Fig.  35. 


M 


Macrocystis  (see  also  "  Giant  Kelp  "), 
68. 


310 


INDEX 


Macrosporaugium  (see  also  "Ovule"), 
145,  146, 150,  151,  159,  160,  161, 162, 
163, 165, 168, 169, 170, 178, 179,  Figs. 
38,  39,  40,  41,  42. 

Macrospore  (see  also  "  Embryo- 
sac"),  145,  146,  148,  149,  150,  151, 
152,  158,  159,  163,  165,  168,  170,  178, 
179,  293 ;  Cycas,  163 ;  Conifers,  165, 
169,  170,  179;  Isoetes,  149,  150; 
Marsilia,  150,  151;  Salvinia,  151: 
Selaginella,  145,  152. 

Madder  family  (Rubiaceae),  215. 

Madrono  (Arbutus  Menziesii),  214, 
240. 

Magnolia,  M.  family,  207,  208,  230, 
235,  237,  238. 

Man  as  agent  in  distribution,  229, 
241,  243. 

Mandevillea  suaveolens,  278,  Fig.  60. 

Mango,  240. 

Mantchuria,  236,  237. 

Manzanita  ( Arctostaphylos) ,  214, 
240,  271,  274. 

Maple,  M.  family  (see  also  "  Acera- 
cese"),  209,240. 

Marattia,  Marattiaceae,  123,  127,  133, 
136, 139, 154, 223, 224 ;  gametophyte 
of,  123, 127 ;  fossil  M.,  154,  223,  224. 

Marchantiaceae,  107,  119. 

Marine  algae  (see  also  "  Sea-weeds  "), 
20,  63,  262,  263,  264,  265. 

Mariposa  lily  (Calochortus) ,  240. 

Maritime  plants,  201,  275. 

Marsilia,  Marsiliaceas,  150,  151,  152, 
155 ;  M.  vestita,  150,  152,  Fig.  39. 

Maruta,  215. 

May-weed  (Maruta),  215,  Fig.  53. 

Mechanical  contrivances  for  cross- 
fertilization,  250,  251,  252,  253,  254, 
255,  295. 

Mechanical  tissues,  263,  267,  296. 

Mediterranean  region,  241. 

Mesembryanihemum,  275. 

Mesocarpus,  60. 

Mesotaenium,  160. 

Mesozoic  formations,  138,  165,  222, 
224,  227,  228. 

Mesquit  (Prosopis) ,  238. 

Metazoa,  25. 


Metzgeria,  106,  119,  Fig.  27. 

Mexican  sage  (Salvia  splendens)t 
249. 

Mexico,  173,  237,  239. 

Micropyle,  171,  181. 

Microspora,  52,  Fig.  8. 

Microspore  (see  also  "Pollen  "),  145, 
149,  150,  159, 162,  163, 170, 180,  203, 
292,  Figs.  39,  40,  42. 

Microsporangium  (see  also  "  Pollen- 
sac"),  145,  146,  159,  161,  163,  165, 
168,  169,  170,  179,  180,  Figs.  38,  39, 
40,  41,  42. 

Mignonette  (Reseda) ,  248. 

Mildew  (see  also  "  Peronosporeae," 
"  Erysipheae  ") ,  82, 91, 92 ;  Rose  M. 
(see  Sphaerotheca) ,  Fig.  24. 

Milkweed,  M.  family  (see  also 
"Asclepias"),  215,  254,  255,  295. 

Mimosa,  24,  212,  283. 

Mimoseae,  212. 

Mint  family  (Labiates),  214,  218,  253. 

Miocene  formations,  227. 

Mistletoe,  85,  205,  278. 

Mitosis  (see  also  "  Karyokinesis  "), 
6. 

Moisture  (a  condition  fqr  growth), 
17,  18,  23,  262,  296. 

Monarda,  249. 

Monera,  4,  31,  32,  33,  45. 

Monk's-hood  ( Aconitum) ,  208,  247. 

Monoblepharis,  83. 

Monocotyledons,  177,  183,  184,  185, 
186,  187,  188,  189,  190, 191, 192, 193, 
194,  195,  196, 197, 198,  199,  200,  201, 
202,  203,  205,  206,  207,  211,  217,  219, 
228,  229,  260,  267,  268,  273,  Figs.  45, 
46,  47. 

Monotropa,  203,  213,  218,  270,  279, 
Figs.  52,  59. 

Monstera,  187. 

Morning-glory,  214,  278. 

Mosses  (see  also  "  Bryophyte,") 
15,  16,  27,  28,  49,  52,  55,  63,  77, 
101,  102,  104,  105,  108,  113,  114, 
115,  116,  117,  118,  119,  122,  123, 
124,  125,  128,  130,  131,  132,  133, 
144,  152,  223,  240,  265,  266,  273,  275, 
283,  285,  286,  289,  290;  Irish  M. 


INDEX 


311 


(see  "Chondrus") ;  Peat  M.  (see 
"  Sphagnum  "). 

Moths,  248,  249. 

Mountains,  229,  231,  232,  233,  234, 
235,  236,  237,  239,  297. 

Mountain  laurel  (Kalmia  latifolia), 
253. 

Movements  of  plants,  3,  22,  23,  24, 
282,  283,  285. 

Mucor,  Mucorini  (see  also  "  Black- 
mould  "),  86,  Fig.  22 ;  Mucor  stolo- 
nifer,  Fig.  22. 

Miiller,  257. 

Musci  (see  also  "True  Mosses"), 
101,  104, 113, 114, 115,  116,  117, 119, 
120;  gametophyte,  114, 115;  sporo- 
phyte,  116, 117 ;  Figs.  29,  30. 

Mushroom,  87,  93,  94,  Fig.  25. 

Mustard  family  (Cruciferas),  209. 

Mycelium,  87,  90,  91,  93,  94,  Fig.  25. 

Mycetozoa  (see  also  "Myxomy- 
cetes,"  "Slime-mould"),  31,  32, 
33,  34,  48,  Fig.  4. 

Mycomycetes  (True  Fungi),  81,  86, 
87,  88,  90,  99,  Figs.  23,  24,  25. 

Myrioblepharis,  883;  zoospores  of, 
84. 

Myrmecophily,  260,  261, 

Myrtaeeae  (see  "Myrtle"). 

Myrtle,  M.  family,  211. 

Myxomycetes  (see  also  "  Slime- 
moulds  "),  31. 


N 


Naias,  Naiadacese,  178,  184,  185, 196, 

198,  Figs.  43,  45. 
Naked-seeded  plants  (see  "Gymno- 

sperms  "). 

Narcissus,  189,  191,  273,  Fig.  46. 
Nasturtium  (Tropoeolum) ,  249,  250, 

251,  Fig.  56. 

Natural  system  of  classification,  12. 
Navicula,  65,  Fig.  15. 
Nectar,  257,  295. 
Nectary,  207,  247,  249,  255. 
Needles  (of  Conifers) ,  168. 
Nelumbo,  207,  Fig.  50. 
Nemophila,  240. 


Nepenthes,  258,  Fig.  58. 

Nereocystis,  68;  JV.  Lutkeana,  68, 
Fig.  17. 

Night-blooming  flowers,  248,  249. 

Nightshade,  214. 

Nitrogen,  12,  36,259,  262,  280,  281. 

Node,  61,  62,  63,  140,  141. 

Non-sexual  reproduction,  25. 

North  America,  232,  233,  236,  237, 
239,  241,  249. 

Northern  Africa,  233. 

Northern  Europe,  230. 

Norway,  230. 

Nostoc,  264,  280. 

Nuclear  division,  5,  6, 56,  62, 179, 286. 

Nuclear  spindle,  6,  Fig.  3. 

Nucleolus,  5. 

Nucleus,  3,  4,  6,  7,  20,  36,  38,  40,  54, 
56,  62,  284. 

Nutation,  23,  282. 

Nutrition,  3. 

Nut  family  (Cupuliferae) ,  206. 

Nymphseaceae  (see  also  "  Water- 
lily  "),  207,  208,  Fig.  50. 


O 


Oak,  237. 

Ocean  (a  factor  in  distribution) ,  234. 

Oceanic  islands,  234. 

Odors  of  flowers,  248,  295. 

CEdogonium,  53,  55,  103,  265,  286, 
Fig.  9. 

Oleander,  21,  215,  271. 

Olive,  215. 

Onagraceae  (Evening-primrose  fam- 
ily), 211. 

Onion,  6,  Fig.  3. 

Onoclea  sensibilis,  237. 

Oogonium,  53,  54,  57,  58,  82,  84,  86 ; 
Chara,  62,  63;  Cystopus,  82,  85; 
QEdogonium,  53;  Saprolegnia,  82; 
Vaucheria,  57,  58. 

Oospore,  53,  54,  57,  85. 

Open  vascular  bundle,  267. 

Operculum  (of  Moss  capsule),  117, 
118,  Fig.  30. 

Ophioglossum,  Ophioglossaceaa,  127, 


312 


INDEX 


132, 133, 134, 141, 154 ;  0.  vulgatum, 

133 ;  0.  pendulum,  133 ;  Fig.  34. 
Orchid,   Orchidaceae,   183,   188,  192, 

194,  195,  190,  197,  198,  234,  254,  255, 

276,  277,  Figs.  47,  57. 
Orchis  spectabilis,  254,  Fig.  57. 
Oregon,  239. 
Organ,  8,  10,  20,  21. 
Organic  substances,  3,  35,  36,  66,  80. 
Oscillaria,  17,  35,  Fig.  5. 
Osmunda,    Osmundaceae,    134,    138, 

139;  0.  cinnamomea,  134;  O.rega- 

lis,  134. 
Ostrich  fern  (Onoclea  struthiopteris), 

126,  129,  Figs.  32,  33. 
Ovary,  161,  177,  178,  179,  181,  189, 

191,   194,  206,   210,  211,   213,  215, 

218. 
Ovule,  28,  159,  160,  161,  163, 165,  170, 

177,  178,  179,  181,  184, 185,  188,  206, 

208,  227,  293. 
Oxalis,  210,  Fig.  51. 
Oxydendrum,  213,  Fig.  52. 
Oxygen,  2,  8,  11,  19,  21,  24,  25,  30. 


Pacific  North  America,  233,  234,  237. 

Pacific  South  America,  237. 

Palaeo-botany,  13. 

Palseopteris,  223. 

Palaeozoic  formations,  222,  223,  224, 
226. 

Palmetto,  187,  238. 

Palms,  187,  197,  198,  228,  230,  234, 
238,  268. 

Pandanus,  Pandanaceae  (see  also 
"  Screw-pine  "),  186,  198,  268. 

Pandorina,  41,  Fig.  6= 

Papaveraceae  (Poppy  family),  209. 

Papilionaceae,  212. 

Pappus,  215. 

Paraphyses,  68. 

Parasites,  30,  80,  81, 82,  85, 88,  89,  90, 
91,  93,  95,  97,  99,  183,  201,  203,  205, 
278,  279,  297;  Algae,  85;  animals, 
88,  90;  flowering  plants,  99,  278, 
279;  Fungi,  81,  82,  84,  87,  88,  91, 
95;  Lichens,  97;  nutrition  of,  297- 


Parsley  family  (UmbeUif eras) ,  211. 

Parthenogenesis,  75,  79,  83;  Chara 
crinita,  75;  Red  Algae,  75,  79; 
Saproleguia,  83. 

Passiflora,  Passiflorinae  (see  "  Pas- 
sion-flower"). 

Passion-flower,  212. 

Pea,  210,  212,  Fig.  51. 

Pea  family  (see  "Leguminosse," 
"Papilionaceae"). 

Peach, 87. 

Pear,  182,  211. 

Peat-mosses  (Sphagnaceae) ,  63,  114, 
115,  Fig.  29. 

Pediastrum,  43,  Fig.  7. 

Pelargonium,  250,  251,  252,  Fig. 
56. 

Penicillium  (see  also  "Blue-mould"), 
92. 

Pepper  family  (Piperineae) ,  206, 208, 
217,  229,  244. 

Perianth  (see  also  "  Floral  enve- 
lope"), 190,  191,  196,  206,  219,  245. 

Peridiueae,  65,  76,  Fig.  15. 

Peridinium  divergens,  65,  Fig.  15. 

Peristome,  29,  118,  283. 

Perithecium,  92. 

Periwinkle  (Vinca),  215. 

Permian  formations,  166,  221,  226, 
227. 

Persimmon  family  (Diospyriuae) , 
214. 

Petal,  161,  177,  179,  185, 186,  202,  205, 
207,  208,  209, 210,  211,  212,  213,  218, 
247. 

Petaloideous  Angiosperms  (Dicoty- 
ledons) ,  217 ;  (Monocotyledons) , 
190,  197. 

Petiole,  201. 

Petunia,  213,  Fig.  52. 

Phseophyceae  (Brown  Algae),  49,64, 
65,  66,  70,  78,  79,  263,  288;  repro- 
duction, 68,  288 ;  structure,  67,  68, 
69 ;  Figs.  17,  18. 

Phalloideae,  242. 

Philodendron,  187. 

Phloem,  124. 

Phlox,  214,  238. 

Phosphorus,  2. 


INDEX 


313 


Photo-synthesis    ( Carbon    assimila- 
tion), 11,  19,23,64,  71. 
Phycocyauin,  37. 
Phycoery  thrin,  71. 
Phycomycetes     (see     also     "  Alga- 

fuugi"),  81,  82,  85,  86,  92,  95,  99, 

288,  Figs.  21,  22. 
Phyllodiura,  271,  274. 
Phylloglossum,  146. 
Phyllosiphon,  85. 
Phytophthora  infestans,  82. 
Pickerel-weed  (Pontederia  cordata) 

189,  Fig.  46. 
Pigments,  22,  49,  64,  263,  274,  297; 

of  Marine  Algae,  64,  263. 
Pig-weed  family    (Chenopodiacese) , 

208. 
Pine,  Pinus,  169,  170,  227,  238,  245 ; 

P.  contorta,  170,  Fig.  42. 
Pineapple,     P,     family     (see     also 

"Bromeliaceae"),,  194,  198,  238. 
Pine-sap  (Monotropa  hypopitys) ,  213, 

270,  Figs.  52,  59. 
Pinguicula,  259. 
Pink,  P.  family  (Caryophyllacese), 

209,  248. 

Pinnularia  viridis,  65,  Fig.  15. 
Piperineae  (see  "  Pepper"). 
Pistia,  271. 
Pistil  (see  also  "Carpel"),  179,  181, 

189,  190,  191, 194,  195,  197,  210,  213, 

215,  216,  245,  250,  253,  254,  256. 
Pisum,  210. 
Pitcher-plant    (see   also    "Darling- 

tonia,"     "Nepenthes,"     "Sarra- 

cenia  "),  205,  209,  258,  Fig.  58. 
Placenta,  181,  206,  209. 
Plankton,  64. 
Plantain   (Musa),   193;    (Plantago), 

241,  251. 

Plasmodium,  23,  31,  32,  34,  Fig,  4. 
Plastids    (see    also    "Chloroplast," 

"  Chromatophore  ") ,  6, 
Pleodorina  Californica,  39,  Fig.  6. 
Pleurococcus,  43,  47,  Fig.  7. 
Pliocene  formations,  227. 
Plum,  93,  165. 
Podophyllum,  236. 
Poinsettia,  210,  248. 


Poisonous  protective  secretions,  260. 

Poison  ivy  (Rhus  toxicodendron) , 
237. 

Polar  nuclei,  179. 

Pollen,  Pollen-spore,  28, 159, 160, 163, 
164,  165,  169,  170,  174,  178, 179,  180, 
181,  195,  203,  250,  251,  252,  253,  254, 
255,  256,  257,  294. 

Pollen-chamber  (of  Cycads),  163, 
Fig.  40. 

Pollen-sac,  159, 160, 169, 179,  202, 293, 
Figs.  40,  41,  42. 

Pollen-tube,  160,  163,  164,  169,  170, 
179, 180,  181,  292,  Figs.  40,  42. 

Pollination,  28,  164, 169, 178, 180, 181, 
189,  190, 194,  206,  244,  245,  246,  247, 
248,  249,  250,  251,  252,  253,  254,  255, 
256 ;  by  birds,  249 ;  by  insects,  245, 
246,  247,  248,  249,  250,  251,  252,  253, 
254,  255,  256;  by  snails,  247. 

Pollinium,  195,  254,  255,  256,  Fig.  56. 

Polycarpicse,  207,  208,  209,  218,  219, 
Fig.  50. 

Polygonacese,  208. 

Polygonum,  206,  Fig.  49. 

Polypodiacese,  138,  151. 

Polypodium,  135,  234;  P.falcatum, 
135,  Fig.  35. 

Polyporus,  87. 

Polysiphonia,  72,  74,  Figs.  19,  20. 

Pomegranate,  211. 

Pond-scum  (see  also  "  Conjugates," 
"  Spirogyra  "),  59,  60,  85,  Fig.  13. 

Pond-weeds  (see  also  "Naiadaceae"), 
28,  178,  186,  198,  Fig.  43. 

Poutederia,  189,  Fig.  46. 

Poplar,  206,  228,  229,  230. 

Poppy,  P.  family  (see  also  Esch- 
scholtzia,"  "  Papaveraceae  "),  209, 
241. 

Potassium,  2. 

Potato-fungus  (Phytophthora  infes- 
tans) ,  82. 

Prairies,  238,  241. 

Prickles,  28,  260,  295. 

Primrose,  P.  family  (Primulinae) , 
213,  214. 

Procarp  (of  Rhodophyceae) ,  74,  75, 
Fig.  20. 


314 


INDEX 


Promycelium,  89,  Fig.  23. 

Proniiba,  257. 

Proteacese,  231. 

Protection  against  animals,  259,  260, 
261,  272,  295. 

Proterandry,  250,  251. 

Proterogyny,  250. 

Prothallium  (see  "  Gametophyte  "). 

Protococcaceas,  42,  43,  44,  45,  46,  47, 
48,  49,  50,  51,  57,  60,  76,  77,  78,  79, 
98,  Fig.  7. 

Protococcus,  280. 

Protomyxa,  32. 

Protonema,  63,  114,  115,  Fig.  29. 

Protophyte,  Protophyta,  16,  37. 

Protoplasm,  3,  4,  22,  23,  61,  72. 

Psilotum,  Psilotacea,  144,  147,  153. 

Pteridophyta,  Pteridophytes  (see 
also  "Ferns"),  16,  120,  122,  123, 
124,  125,  126,  128,  130,  132, 139, 143, 
144, 147,  148,  149,  152,  153,  156,  157, 
158,  159,  1(50,  161,  169,  172,  175, 176, 
184,  196,  223,  225,  268,  273,  290,  291, 
292,  293,  294 ;  embryo  of,  129,  130, 
131;  fossil  P.,  154,  155;  gameto- 
phyte,  123,  126,  127;  heterospory, 
148;  sexual  organs,  128,  129;  spo- 
rangium, 132;  spores,  132;  sporo- 
phyte,  123,  124,  130,  131. 

Pteris  aquilina,  136. 

Puff-ball  (Lycoperdon),  87,  90,  93,  94. 

Pyrenoid,  39,  59,  Figs.  6,  13. 

Pyreuoujycetes,  93. 


Rafflesia,  279. 

Rag-weed  (Ambrosia),  241. 

Rainfall  (a  factor  in  distribution), 

229,  237,  239. 
Ranunculus,  Ranunculaceae,  186,  207, 

208,  217,  246,  247,  270;   R.  aborti- 

vus,    246;    R.    Calif ornicus,  246; 

R.  Purshii,  270,  Figs.  50,  59. 
Rattan-palm,  278. 
Ravenala,  193. 
Ray    flowers   (of  Compositse),  215, 

216. 
Receptacle,  207. 


Red   AlgfE    (Rhodophycea3) ,  49,  63, 

70,  72,  73,  74,  78,  79,  99,  222,  263, 

295,  Figs.  19,  20. 
Red-bud  (Cercis) ,  212. 
Red  cedar  (Juniperus  Virginiana), 

89,  90. 
Redwood    (Sequoia    semper  vir  ens} , 

174,  227. 
Reproduction,  3,  9,  25,  40,  41,  52,  55, 

58,  60,  62,  63,  65,  68,  69,  73,  78,  83, 
104,  105,  264,  285. 

Respiration,  8,  24,  25. 
Resting-spore  (see  also  "  Oospore," 
"Zygospore"),  39,  41,  53,  54,  57, 

59,  60,  63,  264,  265;    Chara,  63; 
Confervacea3,  53,  54;   fresh-water 
Alga3,  264,  265 ;  Fungi,  83,  85,  86 ; 
Vaucheria,  57 ;  Volvox,  41. 

Resurrection-plant  (Selaginella  lepi- 
dophylla) ,  273. 

Rhaphides,  260. 

Rhizoids,  86,  107,  126. 

Rhizome,  193,  264,  282. 

Rhododendron,  214,  240. 

Rhodophycea3  (see  also  "Red  Al- 
ga3  "),  49,  70,  72,  73,  74,  75,  78,  79; 
color  of,  71 ;  continuity  of  proto- 
plasm, 71;  fresh-water  R.,  73;  re- 
production, 73,  74;  Figs.  19,  20. 

Rhus  (see  "  Sumach"). 

Ribes  speciosum,  249. 

Riccia,  103,  106,  109,  110,  111,  118, 
119,  122,  Figs.  26,  27,  28,  33. 

Richardia,  246. 

Ricinus,  200,  209,  Fig.  48. 

Rock-building  Algae,  222. 

Rock-weed  (Fucus) ,  69,  70,  Fig.  18. 

Rocky  Mountains,  232,  237. 

Root-hairs,  281. 

Roots,  21, 30,  67, 68,  69,  113,  120,  122, 
126,  129, 130,  131, 141,  144, 145, 162, 
170,  171, 172,  182,  183, 185,  189, 204, 
259,  267,  268,  276,  277 ;  of  Conifers, 
170,  171,  172;  of  Cycads,  162;  of 
Dicotyledons,  189,  204;  of  Epi- 
phytes, 277;  of  Equisetum,  141 ;  of 
Ferns,  122, 126, 127, 130, 131 ;  Lyco- 
pods,  144,  145 ;  Monocotyledons, 
183, 185,  268. 


INDEX 


315 


Root-stock  (see  "  Rhizome  ") 
Root-tubercles  (of  Leguminosae) ,  280 
Rose  mildew  (Sphaerctfheca) ,  91. 
Rotation  of  protoplasm,  61. 
Royal-fern  (Osmunda  regalis),  134. 
Rubiaceae,  215,  218. 
Ruby-throat  humming-bird,  249. 
Rudbeckia,  241. 
Rushes  (Juncaceae),  189. 
Rusts   (JEcidiomycetes),  88,  89,  94, 

95,  Fig.  23. 
Rye,  93. 

S 

Sac-fungi  (Ascomycetes),  90,  91,  92, 

280,  Fig.  24. 

Sage  (Salvia),  249,  254,  Fig.  57. 
Sage-brush  (Artemisia),  204,  237. 
Sagittaria,  185,  186,  207,  Fig.  45. 
Sago-palm  (see  "  Cycas  revoluta"). 
Sahara,  271. 
St.  Helena,  234. 
Salicoruia,  275. 
Salvia,  253,  254;  S,  pratensis,  254; 

Fig.  57. 

Salvinia,  Salviniaceae,  151,  155. 
Samphire  (see  "Salicornia"). 
Sanguinaria,  200,  Fig.  48. 
Saprolegnia,     Saprolegniaceae    (see 

also    "Water-moulds"),    82,    83, 

Fig.  21. 
Saprophyte,  30,  80,  82, 87,  92,  99, 196, 

203,  205,  278,  279,  297. 
Sarcodes,  205,  279.  , 

Sargassum  (see  also  "  Gulf-weed  ")» 

67,  69,  70,  Fig.  18. 
Sarothamnus,  250,  252,  Fig.  56. 
Sarracenia,  258;   S.  purpurea,  258, 

Fig.  58. 
Sassafras,  230. 

Scale-mosses  (see  "Liverworts"). 
Scales,  125,  165,  170,  195,  270,  277, 

279 ;  epidermal  scales,  125, 195, 277 ; 

leaf-scales,  165,  170,  270,  279,  Fig. 

59. 

Schizomycetes    (see    also    "Bacte- 
ria"), 35. 
Schizophycese  (see  also  "Cyanophy- 

ceas"),  35,  36,45,  284. 


Schizophytes,  Schizophyta,  34,  35, 
37,  44,  47,  284. 

ScitaminefB,  192,  193,  194,  197,  198, 
Fig.  47. 

Scouring-rush  (Equisetum),  139. 

Screw-pine  (Pandauus) ,  186, 198, 228, 
268. 

Scrophulariaceae,  214. 

Sea-anemone,  25. 

Sea-rocket  (Cakile),  275. 

Sea-urchin,  70. 

Sea-weeds  (see  also  "Marine  Al- 
gae"), 20,  63,  64,  287. 

Secondary  growth  of  stems,  142,  147, 
162,  168,  172,  191,  200,  268. 

Sedges  (see  also  "  Cyperaceae  "),  188, 
189. 

Seed,  18,  29,  160,  165,  170,  171,  172, 
183,  200,  243,  244,  245. 

Seed-plants  (see  also  "  Flo.wering 
plants,"  "  Spermatophytes  "),  9, 
10,  14,  16,  21,  23,  28,  30,  156,  158, 
160,  161,  174,  226,  242,  267. 

Seed-vessel,  29. 

Selaginella,  Selaginellacese,  143, 144, 
145,  146,  148, 150, 152,  155,  158,  159, 
163, 169, 171, 175, 225, 226 ;  embryo, 
145 ;  gametophyte,  145, 146 ;  sporo- 
phyte,  146;  S.  lepidophylla,  273; 
Fig.  38. 

Self-pollinated  flowers,  246,  253. 

Sensitive  fern  (Onoclea  sensibilis), 
237. 

Sensitive  plant  (Mimosa  pudica),  24, 
240,  283. 

Sepal,  161,  177,  179,  185,  186,  202, 
206,  207,  208,  209,  211,  213,  218,  246, 
248,  251. 

Sequoia  (see  also  "  Giant  Tree," 
"Redwood"),  167,  174,  227,  230, 
235,  239;  S.  gigantea,  174,  227; 
S.  sempervirens,  174,  227. 

Seta,  116,  120,  124. 

Sexual  cells,  25,  27,  287. 

Sexual  organs,  28,  53,  63,  73,  74,  83, 
106,  108,  116,  128. 

Sexual  reproduction,  25,  41,  44,  52, 
58,  60,  62,  63,  70,  73,  83,  84,  85,  87, 
88,  106,  108,  116,  128,  160,  287;  in 


316 


INDEX 


Algae,  52,  53,  58,  60,  62,  63,  73;  in 
Archegoniates,  104,  105;  in  Fungi, 
83,84,85;  in  Spermatophytes,  160. 

Shield-fern  (Aspidium),  135,  Fig.  35. 

Shooting-star  (Dodecatheon,  217, 
Fig.  52. 

Siberia,  230. 

Sierra  Nevada,  174,  205,  279. 

Sieve-tubes,  124. 

Sigillaria,  147,  226. 

Silene,  206,  Fig.  49. 

Silurian  formations,  174, 221, 222, 223. 

Simplest  forms  of  life,  31,  284. 

Siphoneae,  27,  56,  57,  58,  77,  79,  83, 
99,  221,  222,  288;  fossil  S.,  221,  222; 
structure  of,  56,  57,  58 ;  Figs.  11, 12. 

Sleep  movements,  24,  283. 

Slime-moulds  (Mycetozoa),  3,24,31, 
32,  33,  37,  38,  45,  Fig.  4. 

Smuts  (Ustilagineae) ,  95,  96. 

Snails,  249. 

Snow-plant  (Sar codes  sanguined), 
205,  279. 

Solanumjasminoides,  278,  Fig.  60. 

Sorrel-tree  (Oxydendrum),  213,  Fig. 
52. 

Sorus,  135,  163,  164,  Figs.  35,  40. 

South  America,  231,  237,  239. 

Spadix,  185. 

Spanish  moss  (Tillandsia  usneoides) , 
194,  238,  276. 

Sparganium,  Sparganiacese,  186, 198. 

Spathe,  185,  18(5,  246,  247,  Fig.  55. 

Speedwell  (Veronica),  215,  Fig.  53. 

Spermatophytes,  Spermatophyta  (see 
also  "Flowering  plants,"  "Seed 
plants"),  16,  20,  21,  155,  158,  159, 
160,  164,  184,  292,  293;  classifica- 
tion, 161;  fossil  S.,  226,  227,  228, 
229 ;  structure  of,  158,  159,  160. 

Spermatozoid,  23,  27,  39,  41,  53,  54, 
58,  62,  63,  69,  70,  73,  83,  103,  108, 
119,  128, 129, 140, 144, 145,  146,  149, 
150,  152, 155, 160, 163,  166, 169,  170, 
175,  181,  285,  287,  289,  Figs.  6,  14, 
18,  26,  33,  38,  39,  40. 

Spermatium  (of  Red  Algae),  73,  74, 
75,  Fig.  20. 

Sperm-cell,  103,  150,  160,  169,  181. 


Spermothamnium,  74,  Fig.  20. 

Sphserocarpus,  109,  Fig.  28. 

Sphserotheca,  Ul,  Fig.  24. 

Sphagnum,  Sphagnaceae  (see  also 
"  Peat-mosses  "),  113, 114, 115, 116, 
Fig.  29. 

Sphenophyllese,  225. 

Sphinx  moths,  249. 

Spiderwort  (Tradescantia) ,  4,  Fig.  1. 

Spines,  260,  261,  272. 

Spiraea,  210,  Fig.  51. 

Spirillum  rubrum,  35,  Fig.  5. 

Spirogyra,  59,  60,  265,  Fig.  13. 

Spirotaenia,  60. 

Sporangium,  32, 44,  67,  68,  72,  82,  86, 
94,  125,  132,  133,  134,  135,  137,  140, 
141, 143, 145, 146,  147,  149,  150,  158, 
159,  178,  184,  223 ;  Algae,  67,  68,  72 ; 
Equisetum,  140;  Eusporangiatae, 
132,  133;  Fungi,  82,  86;  Lepto- 
sporangiatae,  134,  135;  Lycopods, 
143  Pteridophytes,  125;  Sperma- 
tophytes, 158,  159. 

Spore,  7,  18,  28,  32,  33,  35,  36,  45,  53, 
54,  63,  73,  75,  82,  83,  84,  87,  107 
109,  110,  117,  125,  133,  135,  139, 
140,  141,  145,  146,  158,  159, 160, 162, 
163, 164, 165,  168,  169,  170, 174,  180, 
203. 

Spore-fruit,  Sporocarp,  72,  75,  77,  87, 
90,  92,  93,  94,  95. 

Sporogenous  tissue,  75,  105,  109,  110, 
111,  112,  117,  125, 132, 133, 134,  135, 
100,  168,  186,  293. 

Sporogonium  (see  also  "  Sporo- 
phyte"),  108,  124. 

Sporophore,  90. 

Sporophyll,  132,  140,  141,  143,  146, 
161,  163,  164,  165,  168, 169, 170, 177, 
244,  293. 

Sporophyte,  55,  77,  105,  108, 109,  110, 
111,  112,  115,  116, 117, 118,  11!),  120, 
121,  122,  123,  125,  126, 129, 130, 131, 
132,  137,  139,  140,  141, 146, 153, 154, 
158,  162,  165,  167,  170,  171, 172,  289, 
290,  291 ;  of  Ferns,  129,  130,  131 ; 
of  Liverworts,  108,  109,  110,  112, 
115,  120,  129 ;  of  Mosses,  116,  117 ; 
of  Spermatophytes,  167,  182. 


INDEX 


31T 


Spur,  207,  215,  250,  251,  254,  255. 

Stamen,  159,  160,  177,  179,  183,  185, 
190,  191,  195, 196, 197,  202,  206,  207, 
209,  210,  211,  214,  215,  216,  244,  250, 
251,  252,  253,  254,  256. 

Starch,  3,  192. 

Starfish,  70. 

Staurastrum,  59,  Fig.  13. 

Stem,  21,  106,  107,  115,  129,  130,  140, 

141,  146,  153,  162, 167,  168,  170, 171, 
172,  182,  189,  191,  192,  200,  201,  204, 
278;    of    Dicotyledons,    200,    201; 
Equisetum,  140,  141;    Ferns,  129, 
130;   Lycopods,  146;   Monocotyle- 
dons, 170,  171;  Mosses,  115;  Pine, 
170. 

Stem-apex  (growing  point),  62,  131, 

140,  141,  170,  171,  185;  Chara,  62; 

Equisetum,   140,   141;   Fern,   131; 

Pine,  170,  171. 
Stemonitis,  32,  Fig.  4. 
Stigma,  178, 179, 181, 185, 194,250, 254. 
Stipule,  201. 
Stolon,  202. 
Stoma,  21,  106, 107, 112, 115, 117, 125, 

141. 

Stornium,  133,  Fig.  35. 
Stonewort  (see  also  "Characese  "), 

22,  103. ' 

Strawberry,  182,  211. 
Strobilus  (see  also  "Cone"),  140, 

142,  143,  146,  164. 
Style,  178. 
Stylophorum,  236. 
Sub-kingdoms  of  plants,  16. 
Sub-polar  zone  of  vegetation,  230. 
Subterranean  stems,  192. 

Sucker  (see   also  "Haustorium "), 

82,  84,  91. 
Sugar,  3. 
Sugar-cane,  188. 
Sulphur,  2. 
Sumach  (Rhus),  237. 
Sundew    (Drosera),    209,    257,    258, 

Fig.  58. 

Sunflower,  216,  241. 
Surf  plants,  263. 
Suspensor  (of  Embryo),  144, 145, 170, 

171,  185. 


Swarm-spores  (see  also  "  Zoo- 
spores"),  23,  24,  34,  38. 

Sweet  pea,  179,  278,  Fig.  60. 

Symbiosis,  279. 

Sympetalse,  202,  212,  213,  214,  215, 
218,  219,  229,  Figs.  52,  53. 

Sympetaly,  218. 

Symplocarpus,  186. 

Synangium  (of  Marattia),  133,  Fig. 
34. 

Synergidse,  179,  181. 


Tapetum,  134,  135,  Fig.  35. 

Tape- worm,  90. 

Tap-root,  162,  168,  183,  200,  204,  268. 

Targionia,  103,  109,  Figs.  26,  28. 

Taxodium  (see  also  "  Bald  Cypress," 

173,227;  T.  distichum,  227;  T.dis- 

tichum  miocenum,  227. 
Taxonomy,  12. 
Taxus  (see  also  "Yew"),  165,  168, 

170,  Fig.  41. 

Tecoma      (see      also       "Trumpet- 
creeper"),  214,  252. 
Temperature,   effect   on   growth  of 

plants,  17. 

Tendrils,  23,  202,  278,  Fig.  60. 
Tentacle  (of  Drosera),  258. 
Tertiary  formations,   173,   174,  227, 

228,  229,  230,  231,  235,  236. 
Tetraspores,  72,  73,  Fig.  19. 
Thallophytes,  16,  48,  80, 101,  220,  221, 

287,  289. 
Thallus,  97-, 
Thaxter,  Prof.  R.,  83. 
Theca  (of  moss-capsule),  117,  Fig.  30. 
Thistle,  215,  216,  241. 
Thorns,  202,  260,  261. 
Tillandsia      (see      also     "  Spanish 

Moss"),   194,  277;    T.   usneoides, 

194. 
Tissues,  8,  10,  14,  21,  87,  124,  183, 

200,  291. 
Tmesipteris,  144. 
Toad-flax  (Linaria) ,  215,  Fig.  53. 
Toadstool,  90,  94. 


318 


INDEX 


Torreya,  227. 

Tracheid,  124. 

Tracheary  tissue,  124. 

Tradescautia,  4,  Fig.  1. 

Trailing  arbutus  (Epigxa  repens), 

214. 
Traveller's  tree  (Ravenala  Madagas- 

cariensis),  193. 
Tree-ferns,  131. 
Tremella,  94,  Fig.  25. 
Tricliia,  32,  Fig.  4. 
Trichina,  90. 

Trichogyne,  74,  75,  Fig.  20. 
Trichomanes,  127,  135,  Fig.  35. 
Tricoccse,  210. 
Trillium,  190,  239. 
Tropics,  231,  234,  240. 
Tropoeolum  (see  also  "  Nasturtium  "), 

249,  250,  251,  Fig.  56. 
True   mosses   (see   also    "Musci"), 

113,  114,  115,  116,  117,  120,  Figs. 

29,  30. 
Trumpet-creeper  (Tecoma  radicans), 

214,  249,  252. 
Tubers,  192,  202,  273. 
Tubercles  (on  roots  of  Leguminosae), 

280. 

Tuberose,  190. 
Tubiflorae,  214,  218. 
Tulip-tree     (see      also     "Lirioden- 

dron  "),  207,  230,  238,  Fig.  50. 
Tunicates,  cellulose  in,  11. 
Twining  stems,  278,  282,  Fig.  60. 
Typhaceae      (see      also      "Cat-tail 

rushes").    186. 


U 


Ulothrix,  52,  Fig.  8. 

Umbel,  212. 

Umbelliferse,  211,  212,  218. 

Unicellular  plants,  10,  20,  284. 

Urn  (Theca),  117,  Fig.  30. 

Uromyces  Caladii,  89,  Fig.  23. 

Utricularia  (see  also  "  Bladder- 
weed"),  108,  204,  258,  259,  270, 
Fig.  58. 


Vacuoles,  7. 

Vallisneria  (see  also  "Eel-grass"), 
22 

Valve  (of  Diatom),  65. 

Vampyrella,  31. 

Vascular  bundles,  113,  124,  134,  140, 
141,  143,  146,  162,  168,  172, 183, 189, 
191,  200,  267,  291 ;  of  Conifers,  168, 
172;  of  Dicotyledons,  200;  Ferns, 
124, 134 ;  Lycopods,  146, 162 ;  Mono- 
cotyledons, 183. 

Vascular  Cryptogams  (see  "Ferns," 
"Pteridophytes"),  122. 

Vascular  plants,  14,  108,  220. 

Vaucheria,  57,  58,  59,  82,  83,  84,  Fig. 
12 ;  reproduction  in,  57,  58 ;  struct- 
ure of  thallus,  57 ;  V.  sessilis,  57, 
Fig.  12. 

Venation  of  leaves,  201. 

Venter  (of  archegonium) ,  102,  103, 
111,  129. 

Venus's  fly-trap  (Dionaea) ,  205,  257. 

Veronica,  215,  Fig.  53. 

Vertebrates,  absence  of  non-sexual 
reproduction  in,  25. 

Vessels,  124. 

Vine  family  ( Vitacere) ,  209. 

Violet,  209. 

Vittaria,  126,  127. 

Volvocacese,  Volvocineae,  27,  38,  39, 
41,  42,  43,  46,  49,  50,  52,  55,  58,  60, 
76,  77,  79,  284,  285,  Fig.  6. 

Volvox,  39,  40,  41,  42,  76,  286,  Fig.  6 ; 
reproduction  of,  41. 


W 


Wall-flower,  210,  Fig.  51. 
Washington,  239. 
Water,  W,  103,  262,  263,  269. 
Water-ferns   (see    also    "  bydropte- 

rides"),  151. 
Water-lily    (see    also    "  Nymphaea- 

ceje"),204,  207,  208. 
Water-moulds  (see  also  ' '  Saproleg- 

nia,  Saprolegniacese  "),  81,  82,  83, 

84,  85,  86,  242,  Fig.  21. 


INDEX 


319 


Water-net    (see    also    "Hydrodic- 

tyon),  43,  44,  Fig.  7. 
Weeds,  240,  241,  243. 
Western  America,  233,  238,  239. 
Western  Asia,  233. 
Western  Europe,  233. 
West  Indies,  194. 
Wheat-rust  (Pucciniagraminis),SS, 

89,  95. 

White  birch,  230. 
White  rust  (Cystopus  candidus),%2, 

83,  84,  85,  Fig.  21. 
Wild-oats  (Avenafatua),  241. 
Willow,  206,  228,  229,  230,  244,  Fig. 

49. 
Wind-pollination  (see  also  "  Anem- 

ophily"),244. 
Winter  buds   (of  deciduous  trees), 

270,  281,  Fig.  59. 
Wood,  woody  tissue,  124,  267. 


Xylem  (see  also  "  Wood  "),  124. 
Xerophytes,  201,  204,  271,  272,  273, 
274. 


Yeast-fungus  (Saccharomyces),  96. 
Yew  (see  also  "Taxus"),  165,  168. 
Yucca,  191,  237,  257,  272. 


Zamia,  Z.  integrifolia,  162, 163,  Fig. 

40. 

Zauschneria,  249. 

Zingiber  (see  also  "  Ginger  "),  193. 
Zoology,  11. 
Zoospores(see  also  "  Swarm-spores  "), 

44,  50,  51,  52,  53,  54,  57,  58,  69,  70, 

73,  78,  82,  83,  84,  86,  102,  104,  118, 

128,  266,  285,  286,  287. 
Zygomorphy,  Zygomorphic  flowers, 

189,  191,  193,  194,  195,  207,  209,  210, 

251,  Figs.  46,  47,  50,  51,  56,  57. 
Zygospores  (of  Conjugatse),  61,  86. 
Zygote  (see  also  "Zygospores),  26, 

39. 


LESSONS  WITH  PLANTS. 

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